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UNITED STATES DEPARTMENT OF THE INTERIOR

IGNEOUS GEOLOGY AND STRUCTURE OF THE MOUNT TAYLOR VOLCANIC FIELD NEW MEXICO

GEOLOGICAL SURVEY PROFESSIONAL PAPER 189— B

IGNEOUS GEOLOGY AND STRUCTURE OF THE MOUNT TAYLOR VOLCANIC FIELD, NEW MEXICO

By CHARLES B. HUNT

ABSTKACT in the Basin and Range part of the field preceded the Mount Taylor eruptions also, but after those eruptions there was renewed The Mount Tftylor volcanic field is situated in northwestern minor folding and faulting in the McCarty syncline, minor New Mexico and embraces a part of the south flank of the San faulting in the gently warped area east of the syneline, and Juan Basin and the adjoining part of the block-faulted Rio continued faulting in the eastern part of the volcanic field- The Grande Valley. Most of the volcanic field is in the Colorado faulting in this part of the area continued during the deposition Plateau province, but the eastern part is in the Basin and Range of the Santa Fe formation and some deposits that are probably province. The boundary between the two provinces is clearly marked topographically and geologically. A variety of land younger than the Santa Fe. forms and several different types of drainage patterns represented INTRODUCTION in the area are due to differences in resistance and structural attitude of the rocks. LOCATION OF THE AREA The sedimentary rocks in the field consist chiefly of Upper Cretaceous sandstone and shale, the Santa Fe formation of late The Mount Taylor volcanic field, named from Mount Miocene and Pliocene age, and some younger deposits of probable Taylor, its largest volcano and most conspicuous topo­ Pleistocene age; also small areas of the Morrison formation, of graphic feature, includes an area of about 2,000 scuare Upper Jurassic age. Volcanic rocks were erupted at different miles in northwestern New Mexico midway between times in this area. Probably the first eruptions were those of the Mount Taylor volcano, in Miocene time, which began with Albuquerque and Gallup. The area lies north of th« rhyolitic tuffs and trachyte, continued with a series of latites, valley of the Rio San Jose. This valley is used by the and concluded with porphyritic andesite. The tuffs, trachyte, Atchison, Topeka & Santa Fe Railway and United and latites apparently came from a central pipe and crater in States Highway 66 for their approach to the Conti­ Mount Taylor, but the porphyritic andesite was erupted mostly nental Divide, which crosses the northwest corner cf the from fissures radiating from the pipe. The tuff and lava erupted by Mount Taylor accumulated in a cone whose top has been volcanic field. (See fig. 3.) considerably lowered by erosion. The cone is estimated to have From the village of Grant, 15 miles southwest of included originally at least 12^ cubic miles of the eruptives. Mount Taylor, the western and southwestern parts of The Mount Taylor eruptions were followed, probably in the volcanic field are easily accessible. A Forest Serv­ Pliocene time, by a large number of basalt sheets erupted by small ice road from Grant extends within 4 miles of Mount volcanoes distributed around Mount Taylor. These sheets of basalt were erupted on erosion surfaces cut around the Mount Taylor Peak. A graded road, not fully shown on Taylor cone. A few of the sheets preceded the latest flows of plate 7, leads from Grant to San Mateo, whence other porphyritic andesite from Mount Taylor, and one preceded roads lead northward along the west side of Mesa the rhyoliUijj- but most of them were erupted after Mount Taylor Chivato and eastward onto the mesa. Laguna and the had become quiescent. The sheets were supplied by many small Laguna Indian pueblo, south of the field, are the best volcanoes, mostly of the central type, although a few fissure eruptions are knojyn. starting points for visiting the east side of Mount Taylor Since the eruptions streams have cut downward nearly 2,000 and Mesa Chivato. Stage roads lead northward as far feet, and the lava sheets now cap high mesas. On these lava- as Marquez. New Mexico State Highway 6, the most capped mesas the volcanoes are marked by cones of lava and direct road west from Albuquerque, crosses the south­ scoriae. Where the lava has been completely removed the old east corner of the area shown on plate 7. The northern volcanoes are marked by isolated necks. The most conspicuous necks are composed almost entirely of basalt; others include parts of the area can best be reached from San Yfidro, basaltic breccia with the basalt, and still others are mostly on State Highway 44, or from Cabezon, which is con­ basaltic breccia. There is practically no deformation of the nected by a fair road with that highway. country rock adjacent to these intrusions. The breccia is believed to be a nonextruded byproduct of a quiet intrusive PRESENT INVESTIGATION process. Folding has been the predominant type of structural deforma­ The field investigation on which this report is l^sed tion of the rocks in the Colorado Plateau portion of the Mount was begun in 1930, with the assistance of D. A, An­ Taylor volcanic field, but faulting has been the predominant drews, R. W. Brown, and W. S. Pike, Jr. In that year type in the Basin and Range portion. The Mount Taylor the portion of the volcanic field north and northwest of eruptions jferb preceded by regional northward tilting, by folding of 'the McCarty syncline, and by folding and associated Mesa Chivato and also a strip about 10 miles wide along faulting west of the syncline and gentle warping east of the the east side of the mesa, including the many volcanic syncline. Probably moist of the displacement along the faults necks, were mapped, In 1931 the field investigation 150009—38 51 52 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 was resumed with the assistance of H. R. Joesting, determinations. During these two seasons several trips R. C. Becker, J. W. Wyckoff, and G. F. Taylor; the were made into the flanks of the Mount Taylor volcano, country along the south side of Mount Taylor and east but only very general relations were noted. The nu­ of the mountain as far as the eastern border of the Upper merous volcanic necks northeast of Mount Taylor were Cretaceous formations was mapped. examined more carefully in connection with the detailed The field investigations of 1930 and 1931 formed a mapping. part of ths Geological Survey's regular program of sur- Returning in 1932, I was able to spend 3 weeks

JD

= ^Sandia I fV!t/s / AMiguerque/

FIGUBE 3.—Index map of New Mexico showing the location of the Mount Taylor volcanic field (stippled) and the approximate boundaries of the physiographic provinces. veys for the claszification of public lands with reference in studying the Mount Taylor volcano and the later to fuel and other mineral resources. The energies of the eruptives immediately surrounding it. I iring this field parties were therefore confined mostly to the map­ study sketch maps were made of the field relations, and ping of the sedimentary rocks. The mapping was done another week was spent hi obtaining additional infor­ by plane-table methods on a scale of 1:62,500 and con­ mation on the volcanic necks east of Mesa Chivato. sisted of careful lateral tracing of formation boundaries The present report therefore includes the results of and carrying altitudes on these horizons for structural both detailed and reconnaissance studies, and in the CONTENTS

Page Abstract.... ______-______-__--___----____-.-.---_.--_------51 Introducticjtr^v- ___--______-_-__----___------_-----__----- 51 Location of the area__-______---___-----_---_---_----- 51 Present investigation---______-______-______-______-__----__-_------_-__---.-- 51 Previous investigations.______-_--_-----_------__----- 53 Geography— ———— ______-___-_--__-- — ———————— __--—— 54 Geomorphic location-______-______..______-----__------_----- 54 Land forms.______54 Drainage ______..______--_---__------_-----_-.----- 55 Climate and vegetation______!______-_-___---_-_--_----_----- 56 Summary o,f the late Tertiary and Quaternary history______---___------___--.----- 56 Chronology of events.--______-______--__------_------~------56 Age alignments.______56 Volcanic activity______57 General chronology.______-______---____-----_----_-_----- 57 The Mount Taylor volcano______..______-_-_-_------__-----_ 58 Rhyolitic tuff.______„______-_---_-__---_------58 Trachyte--______.______.______60 Latite series. ______.._-______--_--__---__-_--_-__----- 61 Porphyritic andesite series.______--______------_-_----__------62 Summary of Mount Taylor eruptions______-______-_____------_------63 Sheet basalt.______.______.______64 Basalt in the amphitheater______.-______-__-____------_--._------65 Vents on the north flank of Mount Taylor______-______-______-___-___-_------_----- 65 Flows on Mesa Chivato______---____-_--_---_------65 La Jara Mesa.______---_--___---_---_--_-_----- 66 Grant Ridges.______— __ 66 Horace Mesa______---_-__--__------67 The volcanic necks______---_-__--_---_-__-_----_- 67 Cabezon Peak______-_--__-__----_------68 Cerro de los Cuates______68 Neck in Gonzales Canyon______68 La Sefiora Peak-______-_--_------__----- 69 Seboyeta Peak_.______69 Basaltic rocks by New Mexico State Highway 6___--__-__-____-_--__-_------_------.------70 Sierra Alesna.______-__-----_-----_-_--_--_----- 70 Other volcanic necks______-__-_--_-____-_---_------70 Basaljp;i_kes-__-______71 Origi^i of the breccia.______72 Summary of basalt eruptions_i______73 Recent flows.______-______r ______-_____-_-_---_-_------73 Structural geology.______73 General structural relations of the volcanic field-______-______-______--_------__------73 Colorado Plateau portion of the field___-______-_------__---_-_------74 McCarty syncline--______.______-____---_-_----___---_-_----- 74 Minor folds within McCarty syncline_-______-______------_------_----- 74 Fo^ch northwest of McCarty syncline______--_-_----__------_------_---_ 75 (i«ii%le warping east of McCarty syncline-______.--___---_------_------75 Basins and Range portion of the field______-______-_-_-_-----_-_------_------76 General structural relations_____.______-______-_--____-_---_---_--_-_-- 76 Faults______._-______---- 76 Folds.-..______---______-_ 76 Age of the deformation_-_--______-______-______----_-_-_---_----- 77 Possible significance of the structural features-______-__-__---_-_------_------_------— 78 Index--.._.______.______79 in ILLUSTRATIONS

PLATE 7. Map showing structure and reconnaissance areal geology of the volcanic rocks of the Mount N. Mex__. ______In pocket 8. A, View of Mount Taylor; B, View looking northeast across erosion surface in Lobo Canyon______'-_ 54 9. Map showing drainage in Mount Taylor volcanic field_-______--___-_____-_-_-__--^-_-_------_-__----- 54 10. A, Porphyritic andesite flows near base of north flank of Mount Taylor; B, V-shaped valley cut in porphyritic andesite. ______54 11. A, Cross section of basaltic neck exposed by erosion in Grant Ridges; B, Cliff face exposing basaltic breccia on rhyolitic tuff near the volcanic neck______-_____--______-_-_-----__---_-_--__--. 54 12. A, Basaltic lava and cinder cone on the sheet basalt capping Horace Mesa; B, View looking northeast f roar,, top of volcanic neck east of La Abra de los Cerros; C, View looking south from Cerro de los Cuates. ______i1.:-- 62 13. A, Cabezon Peak viewed from the bluffs above the village of Cabezon; B, Closer view of south side of Cafcezon Peak showing fairly uniform vertical columns capped with a layer of scoriaceous basalt______^^-_'_. 62 14. A, Volcanic neck east of La Abra de los Cerros; B, Xenolith of fossiliferous Cretaceous sandstone in basaltic breccia in neck in Gonzales Canyon______-____-____--____-__------70 15. A, B, Views of Evans Peak______.______70 16. A, View looking southwest toward Seboyeta Peak; B, Xenoliths of sedimentary rock in breccia along east side of Seboyeta Peak______70 17. A, Dike of basalt and basaltic breccia intruding Cretaceous shale; B, Banded basaltic breccia lying at outer contact of large breccia mass; C, D, Angular unconformity between Santa Fe formation and underlying Cretaceous rocks ______70 18. A, Block diagram showing structural surface of part of Antonio Sedillo Grant at southwest corner of the a^e^; B, Block diagram showing structural surface of most of north hah* of Basin and Range part of Mount Taylor vol­ canic field______-______- 78 19. A, Flat-lying unbroken Santa Fe formation crossing a normal fault in Cretaceous shale; B, Basaltic lava in Fanta Fe formation tilted and broken by small fault______-._____-______---_--_------_-__--_------_---- 78 FIGURE 3. Index map of New Mexico showing location of Mount Taylor volcanic field- ______-______-_-___--___---_ 52 4. Diagrammatic view of part of Mount Taylor volcanic field______54 5. Sketch showing topographic relations in Lobo Canyon-______-___--_--___-_----_--_---_-_--_------.-_--- 55 6. Pedestal of alluvium about 50 feet high left standing in Chico Arroyo, 3 miles west of Cerro de los Cuates_ _ _ _ — _ _ _ 56 7. Arroyo of Rio Puerco 2 miles north of Ojito ranch______-_--_-__-______---___------56 8. Sequence of late Tertiary and Quaternary events in Mount Taylor volcanic field. ______57 9. Hypothetical profile of Mount Taylor___-______-______-______--__--_---__-_---_--___------58 10. Geologic sketch map of amphitheater and surrounding rim of Mount Taylor volcano______59 11. Sketch map showing intrusive basaltic breccia in Gonzales Canyon______-______-_ 69 12. Diagrammatic section through north side of Juan Tafoya Peak______-_-___-__-_-__---_---_-_-_ 71 13. Diagram of columnar jointing in intrusions 4 miles east-northeast of La Sefiora Peak. ______r __ 71 14. Sketch from photograph showing fault relations half a mile west of San Ysidro and 1,000 feet north of Statef ^4gh- way 44 ______^._-_.,_ 77 15. Diagrammatic section illustrating faulting east of Mesa Lucero_-______-__-___--_--_--_---_____---__->- 77 IV IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 53 descriptions an effort is made to indicate the degree of Socorro and Valencia Counties, N. Mex.: Am. Geologist, vol. completeness of the observations. 25, pp. 331-346, pis. 8-60, 1900. Discusses stratigraphy of the Cretaceous strata. PREVIOUS INVESTIGATIONS Johnson, D. W., Block mountains in New Mexico: Am. Geolo­ gist, vol. 31, p. 137, 1903, Briefly mentions block faulting along Numerous reports contain references and descriptions the Rio Puerco in relation to the regional structure bf the Rio of the Mount Taylor volcanic field. The earliest Grande Valley at Albuquerque. describe the general appearance of the area as viewed Reagan, A. B., Geology of the Jemez-Albuquerque region, N. from the nearby routes of travel. Subsequent reports Mex.: Am. Geologist, vol. 31, pp. 67-111, 7 pis., 1903; Indiana give fulled accounts of the geology, but most of them Acad. Sci. Proc., 1902, pp. 187-197, 1903. Briefly mentions Cretaceous stratigraphy of the eastern part of the area in con­ were written to emphasize some specific problem of the nection with regional correlation. area. The reports are summarized as follows: Schrader, F. C., The Durango-Gallup coal field of Colorado Simpson, J. H., Journal of a military reconnaissance from Santa and New Mexico: U. S. Geol. Survey Bull. 285, pp. 2^1-25,8, Fe, N. Mex., to the Navajo country: 31st Gong., 1st sess., S. Ex. 1906. Demonstrates the continuity of the coal field between Doc. 64, pp. 56-138, 146-148, 1850. Mount Taylor is named in Durango, Colo., and Gallup, N. Mex. honor of th^ President, and there is brief mention of the kinds of Johnson, D. W., Volcanic necks of the Mount Taylor region, rock enoouritered along the line of march. N. Mex.: Geol. Soc. America Bull., vol. 18, pp. 303-324, 1907. Marcou, Jules, Re'sume of a geological reconnaissance extend­ Demonstrates that the volcanic necks east of Mesa Chivto are ing * * * t0 the pueblo de los Angeles in California: U. S. correctly interpreted as such and are not remnants of fiSws ®r Pacific R. R. Expl., 33d Gong., 2d sess., S. Ex. Doc. 78, vol. 3, laccoliths. pt. 4, pp. 165-171, 1856. Mount Taylor is identified as a volcanic Shimer, H. W., and Blodgett, M. E., The stratigraphy of the cone. On the basis of fossils Jurassic and Cretaceous ages are Mount Taylor region, N. Mex.: Am. Jour. Sci., 4th se^., vol. assigned to the older rocks exposed in the valley south of Mount 25, pp. 53-67, 1908. Describes and discusses significance of Taylor. fossils collected during the measurement of three seetfins of Marcou, Jules, Geology of North America, Zurich, 1858. This Cretaceous rocks in the Rio Puerco Valley. report contains the first geologic map and cross section of this Gardner, J. H., The coal field between Gallup and San Mateo, part of the country. In the cross section Mount Taylor is shown N. Mex.: U. S. Geol. Survey Bull. 341, pp. 364-378; 190P; The as a volcano. coal field between San Mateo and Cuba, N. Mex.: U. S Geol. Newberry, J. S., Geological report, in Ives, J. C., Report upon Survey Bull. 381, pp. 461-473, 1910. The coal-bearing Mesa- the Colorado River of the West, pt. 3, pp. 65-66, 1861. Contains verde formation is traced from Gallup to Cuba acro^* the a description of Mount Taylor as viewed from the San Jose northern part of the volcanic field. Valley. Darton, N. H., A reconnaissance of northwestern New 1-fexico Newberry, J. S., Geological report, in Macomb, J. N., Report and northern Arizona: U. S. Geol. Survey Bull. 435, p. 69. 1910. of the exploring expedition from Santa Fe, N. Mex., to junction Gives general statements concerning the stratigraphy and struc­ of the Grand and Green Rivers of the great Colorado of the West ture of the eastern part of the Mount Taylor area. in 1859, pp. 9-118, 1876. Describes the appearance of Mount Lee, W. T., Stratigraphy of the coal fields of northern central Taylor as viewed from the northeast corner of the volcanic field. New Mexico: Geol. Soc. America Bull., vol. 23, pp. 62^-629, Howell, E. E., U. S. Geog. and Geol. Surveys W. 100th Mer. 1912. Discusses the regional correlation of the Upper Cretaceous Rept., vol. 3, pp. 227-301, 1875. Recognizes the fact that the formations present in the area. eruptions of the Mount Taylor volcano were later than the folding Stanton, T. W., Some variations in Upper Cretaceous stra­ of the Cretaceous strata and that the sheet basalts were later than tigraphy: Washington Acad. Sci. Jour., vol. 3, pp. 60-61, 1913. Mount Taylor. Also calls attention to faulting in the eastern Discusses the regional correlation of the Upper Cretaceous for­ part of the area. mations present in the area. Dutton, C. E., Mount Taylor and the Zufii Plateau: U. S. Lee, W. T., Relations of the Cretaceous formations to the Geol. Surveiy 6th Ann. Rept., pp. 105-198, 1885. The first com­ Rocky Mountains in Colorado and New Mexico: U. S. Geol. plete desertion of the geology of the volcanic field is in this Survey Prof. Paper 95, pp. 27-58,1916. Discusses the regional report. Discussion is centered around the volcanic rocks, how­ correlation and sedimentation of the Upper Cretaceous forma­ ever, with only passing mention of the sedimentary rocks. In­ tions in this area. cludes a very satisfactory geologic map with topographic contours Lee, W. T., and Knowlton, F. H., Geology and paleontology covering Mount Taylor and the Zufii Mountains. of the Raton Mesa and other regions in Colorado and New Emmons, S. F., and others, Geological guidebook of the Rocky Mexico: U. S. Geol. Survey Prof. Paper 101, pp. 195-198, 1917. Mountain excursion (5th Internat. Geol. Cong., Washington, Discusses the regional correlation of the Upper Cretaceous f®r- 1891), pp. 468-471, John Wiley & Sons, 1893. Includes a de­ mations present in this area. scription, by G. K. Gilbert, of the appearance of the Mount Darton, N. H., Geologic structure of parts of New Mexico: Taylor area as viewed from the Rio San Jose. U. S. Geol. Survey Bull. 726, pp. 257-265, 1922; "Red Beds" Herrick, C. L., The geology of the San Pedro and Albuquerque and associated formations in New Mexico: U. S. Geol. S irvey districts [N. Mex.]: Denison Univ., Sci. Lab., Bull., vol. 11, pp. Bull. 794, pp. 109-137, 1928. Describes the regional structure 93-116, 1898. Discusses stratigraphy of the Cretaceous strata. of the southern part of the area. Herrick, C. L., and Johnson, D. W., The geology of the Al­ Bryan, Kirk, Historic evidence on changes in the channel of buquerque sheet [N. Mex.]: Denison Univ., Sci. Lab., Bull., the Rio Puerco, a tributary of the Rio Grande in New Mexico: vol. 11, art. 9, pp. 175-239, 1900; New Mexico Univ., Hadley Jour. Geology, vol. 36, pp. 265-282, 1928. Evidence shows Lab., Bull., vol. 2, pt. 1, pp. 1-67, 32 pis., with colored geologic that before 1885 the valley floor of the Rio Puerco was e true map, 1900. Discusses stratigraphy and paleontology of the flood plain. Between 1885 and 1890 the channel was deepened Cretaceous &Sta. and widened, a process still continuing. Six small towns have Herrick, C. L.; Reports of a geologic reconnaissance in western been abandoned, and the others impoverished by the erosion, 54 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 GEOGEAPHY Taylor" refers to the whole conical mountain mass and GEOMORFHIG LOCATION thus includes all the peaks in the arc, the amphitheater, and the rugged slopes down to the level of the foothills. The Mount Taylor field overlaps the southeast The highest peak is called Mount Taylor Peak. border of the San Juan Basra and the northwest border Mount Taylor is surrounded by broad, flat-topped of the block-faulted Rio foothills capped with sheet basalt. (See fig. 4.) Mesa ' '•":, 1|§ Grande Valley. (See fig. Chivato, the largest, extends northeastwari for about | s J 3.) It therefore includes 25 miles from the base of Mount Taylor and at most 3 g | portions of both the Colo- places is more than 10 miles wide. From a distance 11 "i rado Plateaus and the Mesa Chivato appears as a nearly flat plateau, but closer 03 °. .u Basin and Range prov- views reveal scores of lava and cinder cones, some a few ^ ~. § inces. The boundary be- hundred feet high, marking the vents that supplied the J-1 § tween these two provinces basaltic lava cap. The top of the. mesa is about 2,000 •§ ^ 1 is clearly marked geologi- feet above the surrounding country and 3,000 feet o » «- cally in the area, a feature +Z rrt *" *^ ' below the peaks on Mount Taylor. Th 8 s of some interest in view of e] t-4 ^} eastern, and western limits of Mesa Chivato are dis­ ° ° g the uncertain boundary tinct geologically and topographically, but the southern | "11 that must generally be limit will be drawn arbitrarily at Water Canyon and P | c? drawn elsewhere in the the canyon that heads southeast of San Mateo. g 2 2 State. The northeastern 03 O IO South of Mount Taylor flat-topped foothills capped § 11 part of the area adjoins F*K with lava, mostly basalt, extend 10 miles from the base 1,3 g the Nacimiento uplift of of the cone to the valley of the Rio San Jose, the princi­ ww| the Southern Rocky pal stream of this part of the area. The^e foothills, D. I g Mountain province. deeply dissected by canyons, are exceedingly rough. LAND FORMS San Fidel Mesa, between Water and Rinconada Canyons, is somewhat lower than Mesa Chivato, but Horace and The most prominent La Jara Mesas, southwest of Mount Taylor, have about topographic feature in the the same altitude as Mesa Chivato and slope gently away area is the mountain for from Mount Taylor. The Grant Ridges are smaller which the field was named. isolated mesas along the northwest side of the valley Viewed from a distance heading northeast of Grant. Mesa Prieta, 15 miles east H>> r . H ^ g ^ 1 M°unt Taylor has a dis- of Mesa Chivato, is another high mesa capped with 11 3 S tinctly conical form, with basalt, but its top is considerably below Mesa Chivato. S ^_ "^ 1 extensive flat-topped foot- The boundaries of the foothills around Mount Taylor 5 i I ® hills spreading away from are located with fair accuracy on plate 7, but no instru­ S ^ .2 1 its base. (See pi. 8, A, and mental control was extended to the tops of the broad I 111 fig. 4.) The top of the foothills. The topography of Mount Taylor as shown •*i M « I cone consists of a series of on plate 7 was sketched without instrumental control 12 '§• 11 peaks distributed along an on the United States Geological Survey reconnaissance * »6 a arc concave to the east. topographic map. The map, as thus mod;fied, repre­ | w 11 Mount Taylor Peak, the sents some improvement over the earlier reconnaissance > I °. .a highest, stands on the west map but still leaves much to be desired in the way of -I g « ^ side of the arc and rises precise locations and delineation of land forms. ^ | s £ about 11,350 feet above sea The lavas capping the mesas are the most resistant §rfo |: o level, a mile higher than the rocks in the area. Erosion of the mesas har progressed

•£•§•§ro ro Rio San Jose 12 miles to almost entirely by removal of the relatively soft sedimen­ ^ I ^ the south. Within the arc tary rocks, thus undercutting the resistant lava caps. 1 -g b- is the crater of the old vol- Erosion by the Rio Puerco has produced a broad valley | §•! cano. The crater has been nearly 2,000 feet below the lava-capped Me^as Chivato enlarged by erosion to form and Prieta. The sedimentary rocks in the valley have P .. 1C fl f £3 an amphitheater covering been eroded to low relief, which emphasizes th e height of a nearly 5 square miles and dozen peaks, volcanic necks of resistant lava, some of about 2,500 feet deep, which rise nearly 2,000 feet above the valley bottom. drained eastward by Water (See pi. 12, 0.) ^ Canyon Creek, which has Elsewhere in the Mount Taylor volcanic field the cut its way through the east surface rocks are entirely sedimentary and consist of 1 S 'S5 O flank of the cone. In this moderately resistant sandstones interbedded with > 03 report the term "Mount thicker bodies of easily eroded shale. The character of GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 8

A. VIEW OF MOUNT TAYLOR FROM A POSITION 25 MILES TO THE SOUTH. Photograph by W. S. Pike, Jr.

B. VIEW LOOKING NORTHEAST ACROSS EROSION SURFACE IN LORO CANYON. This surface, capped with a very thin mantle of gravel, truncates northeastward-dipping beds of sandstone and shale. The high peak in the distance is Mount Taylor.

GEOLOGICAL SUBVEY PEOFESSIONAL PAPEE 189 PLATE 10

A. PORPHYRITIC ANDESITE FLOWS NEAR BASE OF NORTH FLANK OF MOUNT TAYLOR. View looking southwest near center of south line of sec. 9, T. 12 N., R. 7 W.

B. V-SHAPED VALLEY CUT IN PORPHYRITIC ANDESITE, View near center of south line of sec. 31, T. 12 N., R. 7 W. The sky line at the head of the valley marks the rim of the amphitheater of the Mount Taylor volcano. GEOLOGICAL StTRVE? PROFESSIONAL PAPER 189 PLATE 11

A. CROSS SECTION OF BASALTIC NECK EXPOSED BY EROSION IN GRANT RIDGES, 6 MILES NORTHEAST OF GRANT. View looking southwest in sec. 3, T. 11 N., R. 9 W. Basalt in core is jointed in vertical columns and surrounded by a thin zone of platy jointing developed along the contact with white tuff. Bedded breccia overlies the tuff, and basaltic lava overlies the breccia on the far side of the cone. The tuff, chiefly rhyolitic, was erupted from Mount Taylor.

•If

B. CLIFF FACE EXPOSING BASALTIC BRECCIA ON RHYOLITIC TUFF NEAR THE VOLCANIC NECK. In sec. 3, T. 11 N., R. 9 W. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX.

the topography has been controlled by the number, age has been diverted to join a tributary nearer the thickness, and structural attitude of the sandstones. head of the master stream, the Rio San Jose. Northwest and southeast of Mount Taylor, where there DRAINAGE are many thick beds of sandstone with gentle dips, The variety of land forms determines an equal variety there are prominent mesas with steplike benches around of drainage patterns. Plate 9, showing the drrinage them. In the eastern part of the volcanic field a minia­ system in the volcanic field, reveals four conspicuous ture Basin and Range topography has been formed types of pattern, and there are numerous minor varia­ where thick sandstone beds, steeply tilted in successive tions in each type. fault blocks, produce sharp parallel ridges between The Rio San Jose and Rio Puerco are the master valleys eroded in the shale. streams draining the volcanic field. The Rio San Jose Lobo Canyon contains a notable exception to the gen­ flows eastward with an average gradient slightly less eral rule of structural control of the topography; there than 25 feet to the mile and joins the Rio Puerco at an erosion surface that slopes southwest smoothly trun­ the southeast corner of the field. The Rio Puerco flows cates folded beds of sandstone and shale. (See pi. 8, B, southward across the eastern part of the field with an and fig. 5.) A small, inconspicuous remnant of this average gradient of about 20 feet to the mile.

FIGURE 5.—Sketch showing topographic relations in Lobo Canyon. The erosion surface (A) was developed by drainage that formerly went southwest between tie Grant Ridges and Horace Mesa. Lobo Canyon Creek, which now flows aronnd the northwest side of the Grant Ridges, trenches an alluvial plain (B). An alluvial far (C) has been built out on the plain. Hogbacks of eastward-dipping Upper Cretaceous formations (E) protrude above the alluvium. Sheet basalt or andesite caps the high mesas, above which rise basaltic lava cones (D) that mark the vents from which the lava forming the sheets was erupted. surface (A, fig. 5) lies south of Lobo Canyon Creek In the southwest quarter of the field streams vdth. a between the Grant Ridges and Horace Mesa. This radial pattern drain Mount Taylor. The amphithea­ outlier is on the divide between Lobo Canyon and ter, at the hub of the radial pattern, is drained by streams draining to the southwest. Earlier drainage on Water Canyon Creek. This creek receives numerous the surface probably went southwestward from Lobo headward tributaries in the amphitheater, and the arcu­ Canyon, between the Grant Ridges and Horace Mesa. ate divide separating the heads of these tributaries Landslides in the narrow canyon that separates the from the heads of surrounding streams include^1 the Grant Ridges from Horace Mesa may have diverted series of peaks that form the summit of Mount Taylor. the drainage to form Lobo Canyon Creek, which now The valley bottoms of most of these radial streams are flows around the northwest side of the Grant Ridges. relatively wide and flat in the lower courses (pi. 10, A) This hypothesis is suggested by the accumulation of but are commonly narrow and V-shaped toward their lava boulders, some 20 feet in diameter, along the south heads (pi. 10, B}. edge of the outlier and also by the presence of numerous A group of nearly parallel streams flow northeastward landslides along the southwest side of Horace Mesa. on a dip slope of resistant sandstone in T. 15 N., Rs. 8 The conditions are unusual, however, in that the drain- and 9 W. Where the sandstone passes beneath the 56 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 overlying and more easily eroded shale the streams inches.2 There is some variation of rainfall within the unite and meander widely in broad flood plains prior to field, for Mount Taylor rises several thousand feet joining the Chico Arroyo. above the surrounding area and receives numerous local The drainage has a crudely dendritic pattern at the showers that do not extend to the lower country. northwest corner of the volcanic field, on the Pacific The daily range of temperature is considerable. In side of the Continental Divide. A similar pattern is the summer the nights are cool but the days r,re warmed better developed at the northeast corner of the field, by a persistent sunshine. Local rains are frequent in where tributaries of the Rio Puerco and Chico Arroyo the summer, and the temperature may change quickly flow in broad shale valleys between discontinuous sand­ at any locality clouded and showered by a storm. In stone ridges. winter freezing temperatures are reported as common A rectangular drainage pattern has developed in the at night but the abundant sunny days are commonly tributaries of the Rio Puerco in the block-faulted south­ balmy. eastern part of the volcanic field. The river and its Most of the volcanic field is less than 7,500 feet above principal tributaries have, in general, cut across the sea level and is within the Upper Sonorar life zone. structural trends, but the second-order tributaries dis­ Over most of this part of the field sagebrush, the pic­ tinctly follow the fault-line valleys. Where the streams turesque cane cactus, and several other cacti and yuccas flow across the Tertiary deposits, however, there is no are associated with the junipers, nut pines, and grama structural control, because most of these beds are later grass that characterize the life zone. The broad flat- topped foothills around Mount Taylor and most of Mount Taylor itself to an altitude of about 9,500 feet support an open, grassy forest, chiefly of yellow pine,

FIGURE 6.—Pedestal of alluvium about 50 feet high left standing in Chico Arroyo, 3 FIGUKE 7.—Arroyo of Rio Puerco 2 miles north of Ojito ranch. T'le arroyo here is miles west of Cerro de los Cuates. Probably the result of the cutting off of a about 50 feet deep and is gradually being cut laterally under the house built on the meander while tbe arroyo was narrower. Sketch from a photograph. alluvial flood plain. Sketch from a photograph. than the faulting and consist of relatively homogeneous which is characteristic of the Transition zone. Above material in which the stream courses are not controlled. this is the Canadian zone, containing mainly spruces, Nearly all the streams in the Mount Taylor volcanic firs, and aspens. field once flowed on broad alluvial flood plains, but they are now entrenched in arroyos that are cut in the SUMMARY OF THE LATE TERTIARY AND QUATERNARY HISTORY alluvium. Some of the arroyos are 50 feet deep (fig. 6). This erosion has lowered the water table and impover­ CHRONOLOGY OF EVENTS ished the country by impairing the growth of plants The sequence of late Tertiary and Quaternary events dependent on ground water. Bryan * has effectively in the Mount Taylor volcanic field is summarized dia- described the result of arroyo cutting by the Puerco grammatically in figure 8. and pointed out, among other disastrous effects, the AGE ASSIGNMENTS abandonment of six small towns in the eastern part of this volcanic field. The ruins of the settlements are The Santa Fe formation is of considerable assistance still to be seen along the flood plain, and some of the in studying the chronology of events in the Mount houses are being undercut and removed by widening of Taylor volcanic field and provides the only evidence the arroyo (fig. 7). for determining their approximate age. The forma­ tion is generally regarded as of upper Miocene and CLIMATE AND VEGETATION Pliocene age, on the evidence of vertebrate fossils collected by earlier investigators from the formation The climate of the Mount Taylor volcanic field is near Santa Fe. The Santa Fe formation and over­ arid. No measurements of rainfall have been recorded lying younger deposits of probable Pleistocene age very near the field, but the average for western New occupy a belt 20 miles wide adjoining the east border Mexico is generally given as ranging from 10 to 15 of this field and are present in outliers within the field 1 Bryan, Kirk, Historic evidence on changes in the channel of the Rio Puereo, a 2 Bailey, Vernon, Life zones and crop zones of New Mexico: TJ. S. Pept. Agr, tributary of the Bio Orrande in New Mexico: Jour. Geology, vol. 36, p. 281, 192S. North American Fauna, no. 35, p. 9,1913. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 57 near the east border. In this field the sediments of too little for all of Quaternary time and in addition the formation are mostly clastic and probably were an indefinite fraction of late Tertiary time. Accord­ deposited by streams flowing from mountains to the ingly, some uncertainty is introduced regarding the north. The sediments apparently were deposited in age of the beds overlying the Santa Fe in this field, and gradually overlapped the sides of the topographic and these beds may be as young as Pleistocene.

Structural movements Volcanic activity

Mount Taylor *° a-& p: &l?i-di 11 Time of Elevation of drainage around base of Mount Taylor ofandmationy • g,|| DepositionSanof dedifferentiated events cone with respect to present stream levels w .S £ 5X1 § S Sheet basalt -^ .-'o ^•Cfl _a>S S M 2 s" 0> •2 a 2 !jo ® 9 !_ 3 •'S >>> 2 a £ "3 2 5 .a 6c a 6c 3 |g||| H "c3 !i & IIIm

§1 0 ® Drainage at present level si :

Drainage about 100 feet above present level

Drainage about 750 feet above present level

Ol a *

£ Erosion surface about 2,000 feet above present drainage developed around Mount Taylor a a i Irregular topor'-iphy with drainage abort 2.000 <3 feet ab;ve present level I,

''

Probablo irregular topography witli drainage more than 2,000 feet above present level

FIGUKE 8.—Sequence of late Tertiary and Quaternary events in the Mount Taylor volcanic field. The length of line represents approximately the duration of each activity; solid lines indicate fairly definite information; dotted lines indicate considerable uncertainty. Age assignments are based upon the Santa Fe formation and the younger undiflerentiated deposits of Pleistocene (?) age. depression that resulted from the downfaulting of the VOLCANIC ACTIVITY Rio Grande Valley. GENEEAL CHEONOLOGY Probably only the upper part of the Santa Fe forma­ The Mount Taylor volcanic field was the scene of tion overlaps westward into the Mount Taylor field. frequent volcanic activity during late Tertiary time, Locally, thin deposits lie on pediment surfaces sloping probably beginning in the Miocene epoch, and, with south from the Nacimiento Mountains, and these some interruptions, this activity has continued nearly surfaces are in places only 100 feet above the present to the present time. The eruptions were preceded drainage level. The possibility must be considered by the deformation that uplifted the Zufii Mountains that unless the original thickness was far greater than and Mesa Lucero (fig. 3). In the Mount Taylor field that of any of the preserved remnants, only slightly this deformation produced the regional northward more than 100 feet of downcutting has taken place in dip, the folding of the McCarty syncHne, and prol <\bly the region since Santa Fe time. But this is probably most of the other folds and faults associated with that 58 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 syncline. Except for some local basalt flows, the earli­ trachyte, and these were followed by a series1 of porphy­ est eruptions came from the Mount Taylor volcano, ritic latites and trachytes. These early eruptions ap­ the only large volcano in the field. The volcano parently came from a central crater. The final erup­ erupted first rhyolitic tuff and trachyte, next a series of tions of the volcano were porphyritic anc^sites that latites and trachytes, and finally porphyritic aiidesite. were probably supplied mostly by the radiating dikes, These eruptives accumulated to form the cone of RHYOIJETIC TUFF Mount Taylor. The erosion surfaces that were sub­ sequently developed around the cone were flooded with The earliest definite eruptions (p. 59) from the sheets of nonporphyritic basalt and andesite erupted Mount Taylor volcano were pyroclastic materials that, by a large number of vents distributed throughout the so far as known, are now entirely vitric rhyolitic tuffs. volcanic field. A few of these sheets were erupted The tuffs are well exposed in the eastern part of the prior to the latest Mount Taylor eruptions, but most amphitheater and around the outer fringe of the Mount of them were erupted after Mount Taylor had become Taylor cone. quiescent. The sheets commonly overlap the edges In the amphitheater (fig. 10) east of the folded sedi­ of the Mount Taylor cone and spread out on the erosion mentary rocks and north of the creek about 400 feet surfaces sloping from it. The maximum eruptions of of tuff is exposed. The base is concealed, but it is the sheets occurred prior to the final structural move­ probable that the underlying rock is Mesaverde sand­ ments in the field, but there were minor basaltic stone or shale. Most of the tuff is volcanic dust, but eruptions as late as recent time. there is considerable ash and some lapilli. Both the dust and the coarser material are vitric. No undoubted THE MOUNT TAYLOR VOLCANO rock fragments and only a very few crystal?-, probably With the possible exception of some very local erup­ feldspar, were observed. The tuff is distinctly bedded, tions of basalt, the Mount Taylor volcano was the with the coarser material segregated in layer1 . No evi­ earliest and certainly the most active center of vol- dence indicating flowage was found; the exposure is canism in this area. The volcano still retains its conical clearly pyroclastic. The beds in the lowe1" part dip form, but its top has been lowered by erosion. (See about 10° E. Upward the dip increases, and near the fig. 9.) The crater probably covered originally an area top the beds dip 25°. The obvious suggestion of these of less than 1 square mile, but erosion has enlarged it dips is that a cone was being built, but two alternatives seem equally possible—(1) an early tuffaceous cone was built around the crater, and later pyroclastic rocks accumulated on the flanks and beyond the limits of the FIGURE 9.—Hypothetical profile of Mount Taylor showing the probable mass above early cone; or (2) as the top of the exposure is about the present summit that has been removed by erosion. a quarter of a mile north of the base the difference in to an amphitheater covering nearly 5 square miles. dip may be due to location and the result of structural (See fig. 10.) Lavas and tuffs accumulated around the movements. The dips are in the same direction as old crater until a cone at least 3,000 feet high had been those of the sedimentary rocks immediately to the west built. There is no way of determining the original ex­ but are less in amount. tent of these lavas and tuffs, but today they are con­ Tuff that is probably rhyolitic and composed mostly fined to the cone, whose base has a radius of about 8 of dust is locally exposed along each side of Water Can­ miles. (See also p. 65.) yon where the creek is incised into the flanl* of Mount The amphitheater is drained to the east by Water Taylor. About midway between the amphitheater Canyon Creek, which is incised into the east flank of the and the mouth of the canyon 150 feet of tuff is well cone. The rim of the amphitheater is about 2,500 feet exposed beneath the lava that caps the no^th canyon above the creek, and the lavas and tuffs erupted from wall. A short distance upstream from this locality tuff the crater are fairly well exposed along the sides and in is exposed at a lower altitude in the creek bottom. the bottom of the amphitheater. The total thickness in this part of the canyon is prob­ In the eastern part of the amphitheater there is a ably at least 200 feet. About half a mile above the considerable area of folded sedimentary rocks belonging canyon mouth, however, the tuff at the base of the to the Mesaverde formation. The original crater un­ lavas is only 30 feet thick, and near the mouth practi­ doubtedly lay west of this outcrop. The lavas in the cally no tuff is present at this horizon. However, there cone slope in all directions from a point half a mile west is another 100 feet of tuff above the lowest lava. of the junction of the North and South Forks of Water Nearly all the tuff at these outcrops is of dust size. Canyon Creek. A system of dikes radiates from the Locally thin beds of ash consisting of various types of same point, and immediately north of it is the only glass, much of it black, make dark streaks across the breccia found in or on the Mount Taylor volcano. lighter outcrops. Eruptions from this crater began with vitric tuffs, Outcrops similar to those in Water Canyon are found mostly rhyolitic, and at least one lava flow of porphyritic in Lumber Canyon, the next canyon to the southwest. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. HEX. 59 Here, as in Water Canyon, rhyolitic tuff lies at the base tuffs southward from the experiment station is younger of the eruptive series and thins from about 100 feet at than the porphyritic lavas. the head of the canyon to 20 feet at the mouth. Most About 2,000 feet south of the experiment station the of the rhyolitic tuff is of dust size and light-colored. rhyolitic tuff overlies a small basalt flow. Although at The deposit is massive, with lapilli of obsidian and several localities basalt preceded the porphyritic pumice and bombs of pumice scattered irregularly andesite lavas from Mount Taylor, this is the only through it. Nonporphyritic andesite, belonging to the locality found where basalt preceded the rhyolitic tuff. post-Mount Taylor sheet eruptives, overlies the tuff This tuff along the west side of Rinconada Canyon can and is overlain by another 100 feet of tuff. be traced southward nearly to the south line of T. 11 N., A very few feet of rhyolitic tuff, with the base con­ R. 8W. cealed, is exposed beneath the oldest lava along Raton Tuff is excellently exposed at several localities in the Springs Canyon half a mile north of the south line of vicinity of Lobo Canyon, southwest of Mount Taylor.

EXPLANATION VOLCANIC ROCKS

Rhyolitic tuff, porphyri­ tic trachyte and fatite (So/id lines ind/cate rocks that have been identified Ay thin sect/on; c/as/iect'//nes, rocks megrascapica/fy identified}

Mesaverde formation (So/id lines shew areas of/mown outcrops; cfasfied fines ind-'caie probaA/e ac/c/tt/ona/extent) White area in amphitheater con­ tains f&iv outcrops, butprofosxb/y /s composedmostly of porfihy- rit/c /atite. Boundaries between rock t)f>GO are approximate on/y.

Spring z Miles

FIGURE 10.—Geologic sketch map of the amphitheater and surrounding rim of the Mount Taylor volcano. T. 11 N., R. 7 W. It thins out southward, and in the Along the south side of La Jara Mesa, west of Mount mesa to the south younger eruptives rest directly Taylor, near the west line of T. 12 N., R. 8 W., 200 feet upon sedimentary rocks. of tuff, probably rhyolitic, is exposed beneath the cap­ Along the west side of Rinconada Canyon are several ping sheet of andesite as shown in the following section: fine exposures of thick bodies of rhyolitic tuff. Near Section of eruptive rocks on south side of La Jara Mesa the head of the canyon about 500 feet of tuff is overlain Feet by two or more flows of porphyritic andesite, erupted Andesitic lava, apparently a single flow, nonporphyritic; from Mount Taylor. Although exposures are discon­ central part massive; top irregular, scoriaceous, and tinuous, these flows seem not to extend as far south as ropy; bottom 1 foot scoriaceous, with vesicles as much as the experiment station of the New Mexico Agricultural a quarter of an inch in diameter______70 Agglomerate, red, loosely cemented, angular fragments of College. The southward slope of these lavas would scoriacous andesite; fragments small, few of them exceed­ carry them below the top of the tuffs just south of the ing a quarter of an inch in diameter______2J. experiment station, so that possibly the upper part of the Volcanic dust with a little ash; vitric, white, massive_____ 12 60 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

Section of eruptive rocks on south side of La Jara east toward Mount Taylor indicate with some certainty Mesa—Continued that their source was the Mount Taylor volcr.no and not Feet Volcanic ash, white, massive, bombs and lapilli as much as the local vents that supplied the sheet eruptJves. 6 inches in diameter irregularly distributed; bombs and Irregular masses of igneous rock, in part at least of lapilli of pumice, gray glass, and black glass; vitric latitic composition, are associated with the tuff at the throughout ______48 northeast end of the Grant Ridges. The material is Tuff, bedded, white; predominantly dust, but ash, lapilli, poorly exposed, and no means was found to determine and bombs sorted in beds.______.___ 55 Dust, ash, lapilli, and bombs, white, in part at least whether it is a lava flow or an intrusior. At one rhyolitic; generally massive but locally in irregular beds. 92 locality horizontal columnar joints truncated by crude Base concealed by talus. plates are exposed for a width of 6 feet, suggesting There is probably at least 75 feet of tuff concealed by intrusion of a dike, but similar relations were observed the talus below the outcrop. The tuff probably rests in definite flows on the north flank of Mount Taylor. upon sedimentary rocks, for there is little likelihood that It was probably some of this material that WP-S collected a rock as resistant as lava would be concealed in the by J. S. Diller and analyzed by T. M. Chatard in the steep slope of material so easily eroded. A bomb of chemical laboratory of the Geological Surrey.3 The pumice from the lowest unit, analyzed by J. G. Fairchild analysis showed the following composition: in the chemical laboratory of the Geological Survey, Analysis of dacite 6 miles northeast of Grart contains 72 ± 2 percent of SiO2. The index of refraction of the pumice is 1.495 ± 0.005, and the index of the black SiO2 ______49.80 Na2O-___..__ 2.71 MnO___..___ 0.30 A12O3 _------15.33 K2O_---_..__ 4.36 C02 -- —.__. 2.56 /~» obsidian is practically the same. The refractive indices FeO---_-_._ 7.44 H 1 QC of other tuffs examined differ little from this figure, and MgO----_-_ 6.61 TiO2_— ...__ 2.67 101. 08 it is inferred that they are generally rhyolitic. CaO______7. 19 P,OS_____..__ .73 This deposit thins westward, and probably most of the thinning takes place where the lava truncates the Thin beds of light-colored volcanic dust and ash are steepest dips in the sedimentary rocks. The exposed locally interbedded with the sheet eruptives on Mesa tuff is not tilted, like the sedimentary rocks, but lies Chivato. In general these tuffs are less than 10 feet nearly flat. Just how the thinning takes place was not thick. They closely resemble the rhyolitic tuff asso­ determined. Only the youngest tuffs are exposed, and ciated with the Mount Taylor volcano and were the oldest deposits conceivably may be involved in the probably derived from it. folding. But the suggestion is clear that these tuffs are TRACHYTE later than the deformation of the underlying sedimen­ At the junction of the North and South Forks of tary rocks. Water Canyon Creek (fig. 10) a mass of porphyritic About 1% miles east of this locality, just east of the trachyte lies immediately east of the supposed site of cinder cone, tuff is exposed about 300 feet below the the old crater of the volcano. The mass may be an base of the lava. It seems likely that the tuff thickens intrusion that solidified in the eastern part c* the pipe toward the head of the canyon. and crater, but it is improbable that such a p]ug should Abundant rhyolitic tuff is exposed in the sides of the be formed of trachyte and not of latite or andesite, highest mesa in the Grant Ridges, at the south side of which make up most of the cone. If the trachyte is Lobo Canyon. Plate 11, A, shows rhyolite tuff over­ a lava flow, as seems probable, it is older than the flows lain by sheets of basalt and intruded by a plug that and some of the rhyolitic tuff in the northeast rim of contributed to the sheets. The rhyolite tuff is about the amphitheater but is probably younger than the 300 feet thick and rests in nearly horizontal position earliest rhyolite tuffs. A comparatively smrll volume upon tilted sedimentary rocks, although the contact of this early trachyte must have been erupted, for it was not found. Plate 11,5, shows a closer view of the was found at only the one locality mentioned. It was tuff. The material is white, mostly dust, but ash, among the earliest if not the earliest of the lavas, and lapilli, and bombs are sorted in beds. No undoubted the fact that it is intermediate in composition between fragments of sedimentary rock were found. Most of the rhyolite and latite adds interest to its position in the bombs consist of frothy pumice drawn out in in­ the eruptive sequence. timately interwoven thin fibers, but a few are perlitic In the hand specimen the trachyte is li^ht gray, black obsidian. The tuff was not found beneath the speckled with feldspar crystals that are clear except sheet basalt that caps the other mesas of the Grant where stained with yellow or red iron oxide. The Ridges, and, though it may be present and concealed phenocrysts attain a maximum size of about 3 milli­ by talus, the thickness certainly decreases south- meters. Minute vesicles are uniformly distributed. westward. The striking conical form assumed by the Crude flow lines, produced by parallelism of the laths sheet eruptives and the complete absence of such struc­ 3 Clarke, F. W., Analyses of rocks and minerals from the laboratory of the United ture in the tuff, the thinning of the tuff to the southwest, States Geological Survey, 1880-1908: U. S. Geol. Survey Bull. 419, p. 123, analysis D. and the persistence of similar thick tuffs to the north- 1910. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 61 of oligoclase, can be seen in thin sections. The trachyte by porphyritic andesite. The top is about 500 feet is porphyritic, containing phenocrysts of sanidine and above the trachyte, and projection of the dips indicates oligoclase. The proportion of sanidine to oligoclase that near the crater the top of the latite series was about is estimated at about 3 to 1. A few scattered grains 1,000 feet above the trachyte. of quartz may be secondary. Magnetite is finely dis­ These upper lavas of the latite series are porpl yritic seminated through the groundmass and hi a few pheno­ and holocrystalline. They contain about 15 percent of crysts. There is only a trace of ferromagnesian phenocrysts, mostly corroded oligoclase. Magnetite minerals. and altered biotite are less common. The groundmass LATTTE SERIES consists of euhedral sanidine and oligoclase, the former The latite series attains a maximum thickness of predominating, and traces of anhedral magnetite. A about 1,000 feet and makes up a considerable part of the partial analysis of a lava about 400 feet below the top Mount Taylor cone. The series includes chiefly latites, of the latite series showed K2O, 4.92 percent; Na2O, but near the top there are at least one local trachyte 5.78 percent; and CaO, 1.68 percent. flow and some andesite. All these lavas are poorly ex­ The top of the latite series is also locally exposed posed, but a general composite section can be recon­ around the flanks of the cone where valleys have been structed from the several outcrops in the amphitheater. eroded through the later flows of porphyritic andesite. The oldest exposed lava in the latite series crops out Exposures were found at the localities indicated on near the base of the north rim of the amphitheater plate 7, but probably there are many others. about 2 miles east-northeast of Mount Taylor Peak Trachyte is exposed along the Forest Service, trail (fig. 10). The exposure is about 300 feet above the half a mile south of Mount Taylor Peak, where it is trachyte at the junction of the forks of Water Canyon probably overlain by porphyritic andesite, although Creek and about half a mile farther from the crater. the contact was not found. The rock contains The rock is porphyritic. Phenocrysts form about 25 phenocrysts of hypersthene, augite, magnetite, potash percent of it and consist chiefly of sodic oligoclase with feldspar, and andesine, with the potash feldspar predom­ minor quantities of augite and magnetite. The ground- inating. The phenocrysts are 3 millimeters in maxi­ mass is microfelsitic and has an index of refraction mum size and make up about 35 percent of the rock. lower than that of balsam. The groundmass is nearly all sanidine, with very minor Another flow of porphyritic latite, probably slightly quantities of oligoclase, augite, and magnetite. The higher in the series, is exoosed near the base of the rim rock is similar hi composition to the trachyte at the north of the crater. This rock also has about 25 per­ forks of Water Canyon Creek. The trachyte in the cent of phenocrysts, mostly corroded crystals of andes- amphitheater, however, lies below the latite series, ine. Orthoclase, augite, magnetite, and highly altered whereas the trachyte just described is at the top. .This biotite are present in minor amounts as phenocrysts, is the only locality found where the latite series is known and there is a little olivine. The groundmass is a felt- to include trachyte. It is probably a local eruption, like mass of orthoclase and oligoclase in nearly equal because at the other localities examined the top of the amounts and a little magnetite. series is latite. The upper part though not the top of the latite series In the southeastern part of T. 12 N., R. 8 W., in the is exposed in the western part of the amphitheater about canyons 2 miles west of Mount Taylor Peak, a light- half a mile east-northeast of Mount Taylor Peak. This colored lava of vitric latite lies at the top of the series outcrop is nearly 1,000 feet higher than the early and is overlain by porphyritic andesite. The latite is trachyte and is very near the crater. It is porphyritic porphyritic, with phenocrysts of orthoclase, plagioclase latite consisting of about 15 percent of phenocrysts set (probably oligoclase), and biotite in nearly equal in a glassy groundmass. Subhedral sodic oligoclase is amounts and minor quantities of augite and magnetite. the most abundant phenocryst, although euhedral The phenocrysts attain a maximum size of about 3 biotite is common, and there are a few crystals of millimeters and form about 20 percent of the rock. anhedral quartz and magnetite. The glassy ground- The groundmass is mostly glass but includes some mass has an index of refraction lower than that of crystalline oligoclase. balsam. It commonly has a spherulitic structure. The top of the latite series is also exposed in the Irregular masses of latite high in the series are ex­ valleys on the north flank of the cone about IK miles posed on the ridges in the southwestern part of the north-northeast of Lamasca Peak. The exposures are amphitheater. The percentage of potash feldspar is deeply weathered and discolored by hydrous iron oxide. uncertain, because it rarely occurs as phenocrysts and The rock contains about 25 percent of phenocrysts of is nearly indistinguishable in the finely felted ground- sodic andesine and biotite in nearly equal amounts and mass. traces of augite and magnetite. The groundmass The top of the latite series is exposed along the south­ includes oligoclase and orthoclase in nearly equal eastern rim of the amphitheater about 2% miles east- amounts. The overlying flows are porphyritic andesite southeast of Mount Taylor Peak, where it is overlain belonging to the succeeding series. CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

Irregular masses of latite that are poorly exposed with andesite and were probably supplied mostly by the the tuffs at the northeast end of the Grant Ridges may dikes that radiate from the crater and pipe below. be flows of this series. They are described on page 60. Four lines of evidence indicate this derivation. (1) The dike a third of a mile northeast of Mount Taylor With only a few exceptions the dikes are jorphyritic Peak trending nearly northwest for half a mile is the andesite identical in composition with the bulk of the only dike in this area known to be latitic. It intrudes final series of eruptives; (2) no dikes were found to the latite series within the amphitheater but was not intrude the latest flows, although a more complete found to cut the rim or outer flank of the cone. The survey may reveal some; (3) what is believed to be the dike may have supplied some of the later lavas in the crater consists only of breccia, and no central core of latite series. It is porphyritic, with phenocrysts of porphyritic andesite was found; (4) it is very improb­ sanidine, oligoclase, and biotite in nearly equal amounts able that so many dikes would be intruded so near the and a little magnetite. The phenocrysts attain a maxi­ surface without reaching the surface and erupting. mum size of about 3 millimeters and constitute about The dikes are distributed radially within the amphi­ 20 percent of the rock. The groundmass is a felt of theater of the volcano (fig. 10). They are more re­ oligoclase, sanidine, and glass in nearly equal amounts. sistant to erosion than the other rocks in the amphi­ Another dike, probably latitic, less than a mile south theater and stand up as high walls, the highest esti­ of east of Mount Taylor Peak, trends slightly east of mated at 100 feet. north. It intrudes the latite series and may have sup­ Most of the dikes are 20 to 30 feet in width, but a plied some of the later flows in that series. The rock maximum of 60 feet was observed. Within the amphi­ includes about 40 percent of phenocrysts, about half of theater the contact between the dikes and the early which are oligoclase, potash feldspar, and perthite in series of eruptives is generally sharp and marked by a nearly equal amounts. Biotite constitutes about a thin zone of platy jointing and more or less black quarter of the phenocrysts, and the remainder are perlitic glass. Around the rim, however, particularly quartz, hornblende, and magnetite in nearly equal in the southwestern part, where the dikes are in contact amounts. The groundmass consists of glass and a felt with the latest series of eruptives, the contacts are of oligoclase and potash feldspar. commonly very indistinct, and locally th<^ jointed Several rock samples were collected by J. S. Diller structure of the flow and that of the dike grade into from the east side of Mount Taylor, presumably from each other. Horizontal columnar joints are well devel­ the latite series in Water Canyon or the amphitheater. oped in all the dikes observed. So far as could be Analyses of the samples were made T. M. Chatard in determined the dikes stand vertical or nearly so. the chemical laboratory of the Geological Survey and Fifty dikes were found and mapped, and thin sections showed the following composition:4 from 14 were examined. The easternmost dike in the amphitheater, just north of Water Canyon Creek east Analyses of rocks from east side of Mount Taylor of the Mesaverde outcrop, is probably related to the A B C later sheet eruptives and is not described h^re. Two latitic dikes have already been described. Eleven of Si02______68.40 65. 51 65.78 the dikes are very similar in composition, containing Al»0,_ ____-_---__-__-_____-____ 17.99 16.89 17.32 phenocrysts of andesine (rarely oligoclase or labra- Fe_O8_ — — — ______2.66 1.41 3.68 FeO___ — — _ — — — ____ — __ 1.63 2. 52 .46 dorite), biotite, augite, magnetite, and rarely olivine. MgO— — - ______.49 .39 .47 The phenocrysts of augite and magnetite are commonly CaO______.______.67 1. 19 1. 66 Na_O__-_ — ------_ --______4.54 6.42 5.23 in nearly equal amounts, but biotite is the predominant K20___— ______3. 54 5.02 4. 64 mafic mineral. There is generally two or three times H20______.52 . 16 . 14 TiOg.— _ — _ — —— ______.92 .27 as much feldspar as mafic minerals among the pheno­ P_0______.07 . 13 crysts. The groundmass commonly consists of more MnO______.21 .31 .32 or less glass with tiny laths of oligoclase. Where the 100. 65 100. 81 100. 10 oligoclase is anhedral and the groundmass is feltlike there may be some potash feldspar. A. Lava, canyon on east side of San Mateo Mountain. B. Andesite, canyon on east side of San Mateo Mountain. Except in the bottoms of some of the largest valleys, C. Quartz latite, canyon east side of San Mateo Mountain. flows of the porphyritic andesite series form the surface of most of the flanks of the Mount Taylor cone. This POKPHYRITIC ANDESITE SERIES series apparently constitutes a smaller volume of erup­ Probably all the eruptives described above, except tives than the latite series. possibly the latest flows, were poured out from the One of the thickest accumulations of thes^ andesitic central vent. The final series of eruptions of the lavas is along the south rim of the amphitheater, 2 Mount Taylor volcano consisted chiefly of porphyritic miles southeast of Mount Taylor Peak. The thick­ ness here is estimated at 500 feet and represents several • Clarke, F. W., Analyses of rocks and minerals from the laboratory of the United States Geological Survey; 1880-1908: U. S. Qeol. Survey Bull. 419, p. 123, 1910. flows, certainly not less than three. Exposures are not GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 12

A. BASALTIC LAVA AND CINDER CONE ON THE SHEET BASALT CAPPING HORACE MESA. View in center of sec. 3, T. 11 N., R. 8 W.

B. VIEW LOOKING NORTHEAST FROM TOP OF 'J7 ' ANIC NECK EAST OF LA ABRA DE LOS CERROS. Cabezon Peak (right) is composed almost wholly of basalt. Cerro Chato (left) is mostly basaltic breccia. Road in foreground serves as scale for nearby buttes of Cretaceous shale.

C. VIEW LOOKING SOUTH FROM CERRO DE LOS CUATES. The mesa across the Rio Puerco is capped with'Cretaceous sandstone. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 13

A. CABEZON PEAK VIEWED FROM THE BLUFFS ABOVE THE VILLAGE OF CABEZON. The peak is a volcanic neck of basalt jointed in nearly vertical columns but capped with highly scoriaceous basalt. Photograph by W. T. Lee.

B. CLOSER VIEW OF SOUTH SIDE OF CABEZON PEAK. Showing fairly uniform vertical columns capped with a layer of scoriaceous basalt at the top of the neck. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELp, N. MEX. 63

sufficiently good to determine with certainty the limits Along the west side of Kinconada Canyon the por­ of individual flows. They rest upon porphyritic latite phyritic andesites are overlapped by the sheet eruptives. of the preceding series. Thin sections of three flows (See pi. 7.) At the head of the canyon the porphyritic show very similar composition. Each includes about andesite series consists of two flows resting upon rhyo- 35 percent of phenocrysts, mostly of andesine but with litic tuffs. The lower flow, about 100 feet thick, has minor amounts of augite and magnetite. One rock very irregular columnar jointing and is not markedly has a few olivine pheiiocrysts. The groundmass is scoriaceous. It contains only a few large corroded practically all tiny laths of oligoclase with a little glass. phenocrysts of calcic andesine set in a felted grouper A sample of the highest flow was analyzed for alkali mass of feldspar and glass. There is little magi;ej$te content by E. T. Erickson in the chemical laboratory of or ferromagnesian mineral. The material has a distinct the Geological Survey, with the following result: K2O, flow structure. The upper flow is highly scoriaceous 3.42 percent; Na2O, 5.62 percent; CaO, 4.00 percent. and about 40 feet thick. It is abundantly porphyritic, Half a mile southeast of Mount Taylor Peak andesite with many euhedral phenocrysts of calcic andesine $nd is exposed above the latite series. The contact with minor quantities of olivine, augite, and magnetite, ja a the trachyte, here the top of the latite series (p. 61), was groundmass of dark glass. There is only a very sliglit not found. At least two flows are present. The lower suggestion of flow structure. one is nonporphyritic and estimated to include about Porphyritic andesite, interpreted as derived from 75 percent of oligoclase, 20 percent of glass, and 5 per­ Mount Taylor, is exposed beneath later sheet erupfives cent of magnetite. There are few crystals of ferro- in Raton Springs Canyon. The rock approaches a rnagnesian minerals. Some crystals of oligoclase are basalt, having phenocrysts of labradorite, but the fairly large, but there is complete gradation from these groundmass is a less calcic plagioclase. Olivine and to microcrystalline size. The younger andesite is dis­ augite are subordinate as phenocrysts; augite and mag­ tinctly porphyritic, with about 15 percent of pheno- netite are common in the groundmass. crysts, mostly of sodic andesine and minor quantities Where Lumber Canyon and Water Canyon cut the of augite and only traces of magnetite. Most of the southeast fringe of the Mount Taylor cone the ap­ rock consists of tiny laths of oligoclase in dark glass. parent order of eruptives is reversed. PorphjTitic Mount Taylor Peak, at the west rim of the amphi­ andesite, believed to have been erupted from Mount theater, is capped with similar porphyritic andesite. Taylor, overlies nonporphyritic andesite that may have Probably the flows were once connected but have been come from the plug of similar andesite in the eastern part separated by erosion. of the amphitheater. The nonporphyritic andesite is A valley north of Mount Taylor Peak exposes the probably related to the generally later sheet eruptives. top of the latite series. Lamasca Peak, next to the The porphyritic andesite contains phenocrysts, chiefly northeast, is capped by flows which megascopically of calcic andesine, with traces of olivine, augite, and resemble the porphyritic andesite series. magnetite. The groundmass is mostly calcic oligoclase Another valley at the rim east of Lamasca Peak with minor and nearly equal quantities of augite, olivine, exposes lavas that are probably latitic and may be con­ nected with the exposed top of the latite series \% and magnetite.

miles north-northeast of Lamasca Peak (p. 61). Por­ SUMMARY OP MOUNT TAYLOR ERUPTIONS phyritic andesite caps the ridge east of the valley and covers most of the north and east sides of the cone, but The eruptions of the Mount Taylor volcano began the latite may be exposed in the deeper canyons in the probably in late Miocene time, subsequent to the major part of the cone mapped as porphyritic andesite. movements of folding and faulting in the central andu Three thin sections from these flows indicate little vari­ western part of the volcanic field. (See fig. 8.?) Tlie ation in the lithology. The rocks are porphyritic, and volcano broke out in the deepest part of the McCarty most of the phenocrysts are andesine, but there are syncline, which was the center of maximum intensity minor quantities of augite and magnetite and traces of of the early folding. Inasmuch as both the volcano hypersthene and biotite. The groundmass makes up and the fold are unusual features of the southera San about 65 percent of the rock and consists of felted Juan Basin, their association seems to indicate more feldspar and glass. Oligoclase can ordinarily be recog­ than fortuitous coincidence. The volcanic activity nized in the groundmass, but considerable potash feld­ followed the major structural movements, but the time spar is probably occult in the glass. interval is not known. It is interesting, though purely Much later sheets of basalt and andesite from ex­ speculative, to consider the possibility that the struc­ traneous sources overlap the edges of the cone on the tural movements and fusion of the deep-seated magma north, east, and west sides. There are similar overlaps were contemporaneous and related, and that the time along the south side, but the numerous canyons there interval between the structural movements and surface provide opportunity for examining the outer fringe of eruptions was that required for the lava to find its way flows from Mount Taylor. to the surface. 150009—38———2 64 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

The eruptions occurred in a fairly well defined se­ erupted from central vents, though a few came from quence of composition. The earliest eruptions were fissures. Where the sheets are still preserved as the more or less violent ejections of rhyolitic tuffs. Next caps of mesas the vents are marked by conical piles of followed relatively quiet eruptions of porphyritic lavas lava and cinders as much as 500 feet high. (See pi. 12, that are separable into two and possibly three series A.) Where erosion has removed the lava cap and on the basis of their content of potash feldspar.. The lowered the country rock the vents are marked by porphyritic trachyte, interpreted as a flow (p. 60), picturesque necks, columns of lava and breccia that probably followed much of the rhyolitic tuff but has solidified in the pipes without reaching the surface. only a small volume. This was followed by a large (See pis. 12, B, C; 13, A; 16, A.) Locally a relief as volume of porphyritic latite—interrupted, however, by great as 1,800 feet has been produced around lava at least one more flow of porphyritic trachyte. The necks. The breccia necks (pi. 12, B} are less resistant latite hi turn was followed by a slightly smaller volume and topographically are not so prominent. Several of of porphyritic andesite. The following table gives the necks are elongate and dikelike; others are appar­ estimates of the relative volumes of these materials: ently cylindrical. Johnson 5 showed conclusively that the irterpretation Estimated volume of Mount Taylor eruptives of the necks as such is inescapable. Vert;cal contacts Maximum Minimum Estimated with platy jointing can commonly be found. In addi­ observed area cov­ volume Rock series thickness ered (square (cubic tion, erosion has produced all gradations from the lava (feet) miles) miles) cones, with the lava sheets completely around the vents (pi. 12, A), through cones with the lava removed from Rhvolitic tuff ______500 155 5. 1 only one side, exposing the pipe (pi. 11, A), to necks Trachyte. ______(?) . 1 .05? 1,000 60 4.0 from which the surrounding sheets of lava have been Porphyritic andesite series _ _ - 500 11Q 3.6 completely removed (pis. 12, B, C; 13, A', 15, A; 16, A). Most of the vents are northeast of Mount Taylor, The figures indicate that a minimum total volume but a few are scattered around the south and west sides. of about 12}o cubic miles of lava and tuff was erupted The old cone is nearly surrounded by the sheets, which, from the Mount Taylor volcano, a volume sufficient on all sides except the southeast, overlap the earlier to provide a 6-inch blanket over the entire State of porphyritic lavas from Mount Taylor. Some of the New Mexico. lava sheets broke out from vents on the flanks of the Probably the rhyolite and trachyte and most of the Mount Taylor volcano, adding their lavas to the cone. latite were erupted from the central crater, whereas Numerous dikes were intruded at the time of the most of the porphyritic andesite and possibly the sheet eruptions. Most of them trend es.st of north, youngest latite were erupted by dikes intruded radially nearly parallel to the strike of the faults in the eastern around the main pipe. The eruptives now make a part of the volcanic field and to the strike of one set of cone 3,000 feet high with a base covering about 150 faults in the central and western parts. However, only square miles. The cone is what is left of the volcano one dike was definitely intruded along a fault surface. after prolonged erosion has accomplished about 2,000 The necks of dikelike form generally trend east of north, feet of valley cutting in the surrounding country. but no systematic distribution of these necks was Certainly the original volcano was higher and probably determined. The alinement of Sleeping Found Butte, covered a larger area than the preserved remnant. Juan Tafoya Peak, and Santa Rosa Peal' is obvious, Figure 9 shows the probable minimum amount of but the trend of this alinement is nearly north-south, erosion that has altered the cone. The figure indicates inclined 15°-20° to other regional trends. Detailed ,i lowering of the top by nearly 1,000 feet but does not mapping of the cones on Mesa Chivato might provide consider the possibility of lowering of the flanks. Dur­ the clue for determining a system to the location of the ing this period of erosion successive pediments, or volcanoes, for the vents there are more closely spaced erosion surfaces, were developed around the cone. It than elsewhere. was mostly on these surfaces that the sheet basalts were The basaltic lavas erupted by these volcanoes flowed erupted, and these later lavas have proved so resistant as sheets. They generally contain a very few pheno- to erosion that several of the old surfaces are still pre­ crysts, mostly olivine. The feldspar in the lavas is served under the lava capping. generally either calcic andesine or sodic labradorite, and, depending upon which predominates, the indivi­ SHEET BASALT dual flows are designated respectively ande^te or basalt. Some of the sheet eruptives preceded the latest flows There is commonly a glassy groundmass, however, and from Mount Taylor, but most of them were poured out the feldspar of this groundmass is presumably less after Mount Taylor had become quiescent. The sheets calcic than the larger, determinable crystals. More spread out on erosion surfaces developed around the 5 Johnson, D. W., Volcanic necks of the Mount Taylor region, N. Mex.: Geol. base of the Mount Taylor cone, and most of them were Soc. America Bull., vol. 18, pp. 303-321,1907. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 65 comprehensive studies, therefore, may prove that the lor. Just southwest of the Fernandez camp there are basaltic type is less abundant than is indicated in this probably six or seven flows. These lavas thin yest- report. This lava series is loosely designated basaltic ward (and probably eastward), and in two canyons the to avoid confusing it with the porphyritic andesite underlying porphyritic andesite is exposed. series erupted by Mount Taylor. Tremendous quan­ These basaltic lavas are all dark gray and dense. tities of basaltic breccia in the pipes and in some of the The more scoriaceous portions of the flows weather red, dikes consist of fragments of basalt and sedimentary apparently because of oxidation of the abundant magae- rocks in a friable glassy matrix. Apparently very little tite. There are a few large phenocrysts of olivine, the of the breccia was extruded. A few thin beds of tuff, only megascopic mineral in the flows. The rocks .con­ probably rhyolitic, are interbedded with some of the sist of sodic labradorite or calcic andesme, olirifle, lava sheets on Mesa Chivato. This tuff may have augite, and magnetite, with more or less glass. The been a result of minor explosions of the Mount Taylor plagioclase generally forms more than 50 percent of the volcano near the end of its activity. rock, and olivine is generally the most common mafic

BASALT IN THE AMPHITHEATER mineral. Magnetite ranges from a trace to about 10 percent. In the eastern part of the amphitheater (fig. 10) a FLOWS ON MESA CHIVATO mass of basalt intrudes folded sedimentary rocks. The intrusion is bordered on three sides by platy joints, but Mesa Chivato is a part of the highest and no doubt near the top the central core consists of highly scoria- oldest pediment developed around the Mount Taylor ceous basalt. The dip of the platy jointing converges cone, and many small volcanoes flooded it with street upward, suggesting that the intrusion narrowed up­ basalt. Probably these eruptions occurred soon riter ward, but study of jointing in the necks northeast of the pediment was formed, because the smooth surface Mount Taylor discounts this evidence. For example, beveling resistant sandstone and easily eroded shale the zone of platy jointing on the north side of the in­ would not otherwise be preserved. trusive mass dips 55° N., but it may have developed No detailed work was done over most of the interior by consolidation of the basalt against earlier intrusives, of Mesa Chivato, and neither the number nor the exact as basaltic breccia. Such relations are common in location of the volcanoes is known. About 50 cones many of the necks, and so the jointing cannot be re­ were counted in a view from the north flank of Mount garded as necessarily indicative of the limits and shape Taylor. They seem more or less clustered, with of the whole intrusion. broad lava flats separating them. Each of the few Whether or not the intrusion domed the sediments cones examined is composed of basaltic lava and cinders cannot be determined. Elsewhere in this field the in­ of scoria. No craters were found. trusion of these basaltic rocks did not deform the coun­ Some vitric and some lithic tuffs commonly underlie try rock. the basaltic sheets, and locally pumiceous tuff is irter- The intrusive mass is olivine basalt in which small bedded with the flows. The source of the tuffs is not phenocrysts of olivine and augite form about 10 percent known. The lithic tuffs were probably erupted by the of the rock. Augite but not olivine is present in the vsnts that supplied the basaltic sheets, for lapilli groundmass, which contains also about three times as of scoriaceous basalt are common, and such material much labradorite as augite and minor quantities of was not erupted by the Mount Taylor volcano. How­ magnetite. ever, tlie pumice is apparently rhyolitic and wpuld This intrusion may mark the source of some of the represent a separate, extreme differentiate if erupted olivine andesite sheets that underlie the latest porphy­ by these vents. It may have been thrown out du*ing ritic andesite on the southeast flank of Mount Taylor the final stages of the Mount Taylor volcano or may (p. 63). But the careful tracing of individual flows have an extraneous source. necessary to decide this point was not attempted during These sheet eruptives include both andesite and this study. basalt. At Garter Mesa, 2K miles southeast of Mar- quez, the lava is andesitic, being composed rougljly of VENTS ON THE NORTH FLANK OF MOUNT TAYLOR 40 percent of calcic andesme, 10 percent of olivine, Eruptions broke out from several vents on the north­ 10 percent of augite, 5 percent of magnetite, and east flank of the Mount Taylor cone. Basaltic lava 30 percent of glass. There are a few large pheno­ and cinder cones were built upon the earlier porphyritic crysts of olivine. At the southeast corner of Mesa andesite flows of the volcano, and tongues of basalt Chivato, 4% miles east of Picacho Peak, are two basaltic were poured northward down the slope. These erup- flows. Both have distinct amygdules coated ydth tives are younger than the porphyritic andesites, but zeolites. The lower flow is composed roughly of 65 their age relative to the sheets on Mesa Chivato is not percent of labradorite, 15 percent of olivine, traces known. At least 300 feet of lava from these vents of magnetite, and glass. The upper flow is competed accumulated locally on the lower flanks of Mount Tay­ roughly of 50 percent of sodic labradorite, 5 percent of 66 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 magnetite, and a feltlike mass of glass and tiny anhe- edge of the Mount Taylor cone, and entered some of dral crystals of mafic minerals. the valleys eroded in its flanks. In the eastern part of Andesite occurs in the lower part of Canyon Colo­ the mesa the sheets probably overlap porphyritic andes­ rado, near the north base of the Mount Taylor cone. ite erupted by Mount Taylor. Elsewhere they rest The flow apparently came from a cone just west of the upon the rhyolitic tuff, except in the western part of creek. The flows slope 5° S. on the south side of the the mesa, where they cover the truncated edges of the cone and extend a few hundred feet up the creek, over­ folded sedimentary rocks. lapping the eroded flows of porphyritic andesite. To In the southern part of the mesa, near the west line the north, the flowTs slope nearly 12° near the cone but of T. 12 N., R. 8 W., the lava is dark-gray hypersthene flatten downstream and extend onto Mesa Chivato. andesite. The main part of the flows is de-nse, but the The flows are andesitic, dark gray, and nonporphyritic. upper part and bottom are highly scoriaceous. The The composition is roughly 75 percent of calcic andes- dense portions of the flows are jointed in vertical col­ ine, 10 percent of olivine, 10 percent of augite, and 5 umns. The thickness of individual flows ranges from percent of magnetite. about 50 to about 80 feet. The rock is composed of Erosion around the edges of Mesa Chivato has locally about 40 percent calcic of andesine, 10 percent of hy­ exposed the roots of cinder cones and has therefore pro­ persthene, 5 percent of magnetite, and 45 percent of duced natural cross sections of these volcanoes. At one cryptocrystalline groundmass. of these, near the head of Marquez Canyon, the exposure GRANT RIDGES shows an inner core of dense basalt locally chilled against an outer zone of basaltic breccia. The plug is 800 to The Grant Ridges are three lava-capped mesas north 1,000 feet in diameter, and the exposed section is about of Grant and southwest of Lobo Canyor. The lava 600 feet high. It intruded thick sandstone and shale sheets on these mesas are basaltic, were undoubtedly of the Mesaverde formation, built a cone on the high connected, and may have been continuous with the surface of the mesa, and contributed to the lavas on basaltic lavas that cap Horace Mesa and possibly La that surface. Undisturbed and unaltered sandstone of Jara Mesa. The lavas on the ridges were supplied by the Mesaverde is exposed 30 feet from the breccia, but at least four vents on the most northeasterly mesa. the contact is concealed. The dense central core grades One of the vents is located along the southeast side of upward to scoriaceous and even ropy basaltic lavas that the mesa and provides one of the most instructive cap the mesa. The scoriaceous parts of the outcrop are exposures in the volcanic field. commonly weathered to brick red, but the dense por­ At this exposure erosion has developed a natural tion is deep gray. The core is jointed in columns that, cross section of the volcano (pi. 11, A). Here a cylin­ near the base of the exposure, dip 30° towrard the outer drical pipe of basalt about 500 feet in diameter intruded contact. Upward the columns are masked, but to judge the exposed tuffs and erupted onto the erosion surface from other localities they probably assume a nearly ver­ cut on the tuff. Within the pipe basalt in contact with tical position. The columnar jointing gives way to tuff is jointed in plates to a depth of a few inches. The platy jointing at 3 to 5 inches in from, the contact with remainder of the basalt is jointed in vertical columns. basaltic breccia. The contact between the platy-jointed These relations are common in the Mount Taylor basalt and the breccia is sharp but is crenulated for a volcanic field and suggest that cooling in the pipes width of 2% inches, whereas the boundary between the progressed downward from the craters rather than platy jointing and the columnar jointing is regular and horizontally from, the contacts. The flat-lying beds of does not follow these crenulations. The breccia con­ tuff at the locality shown in plate 11, A, illustrate the sists of an irregular assemblage of ropy, scoriaceous general absence of structural deformation accompany­ basalt, dejise basalt, and boulders of sedimentary rock ing the basalt intrusions in this volcanic field. Some in an extremely friable glassy matrix. Boulders of important exceptions, however, are noted in descrip­ sandstone are also present in the core cf the plug, but tions of other localities. Above the tuff can be seen a they are less abundant toward the center and top. cross section of the cone, which is composed of scoria and short flows of basaltic lava. The scoria consists of LA JARA MESA basaltic boulders, mostly less than a foot in diameter, The pediment north of Mount Taylor was probably in a friable glassy matrix. A very few small fragments continuous around the west side of the mountain and of sedimentary rocks were found with the scoria. Ex­ across La Jara Mesa. Near Mount Taylor the pedi­ ceedingly fluid basalt, erupted after the scoria, flowed ment is cut across rhyolitic tuffs and porphyritic andes­ southwest in sheets less than 100 feet thick for at least ite erupted by Mount Taylor, but to the west the sur­ 7 miles down a \% percent grade. face truncates folded sedimentary rocks. Four vents At the other vents kt the Grant Ridges thin bands of in the southern part of the mesa supplied most of the perlitic glass, locally squeezed into mhiute isoclinal basaltic lava cap. These lavas spread out in sheets, folds, form the contact between the basrlt intrusions flowed against the erosion scarp developed at the west and the earlier rhyolitic tuff. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 67

The lava flows are mcstly basalt consisting of about dike is exposed reaching the erosion surface on which it 30 percent of sodic labradorite, 5 to 10 percent of was erupted. An elongate cone has been built, marked olivine, 10 percent of augite, and 5 percent of magnetite chiefly by highly scoriaceous basalt and loose cinders. in a cryptocrystalline and glassy groundmass. Some Apparently only a single flow was supplied by the andesite was found which consisted of about 60 percent fissure. Three-quarters of a mile south of this locality of calcic andesine, 5 percent of olivine, 5 percent of there is another exposure of basalt, probably eruf 4/ed augite, and 5 percent of magnetite in a cryptocrystalline along the same fissure. At this exposure there are two groundmass. cones, each about 250 feet high. Several basalt samples were collected by J. S. Diller Just east of the northernmost exposure of the erup­ "6 miles northeast of Grant",6 presumably from the tive is an outcrop of basalt interpreted as part of an northeasterly mesa of the Grant Kidges. They were early flow. The outcrop is1 about 200 feet wide on an analyzed by T. M. Chatard in the chemical laboratory eastward-trending promontory. The basalt lies upon of the Geological Survey with the following results: 6 feet of light-colored tuff, which in turn overlies Cretaceous sandstone. The tuff has been baked brick Analyses of basalt from Grant Ridges red by the basalt. The flow is about 30 feet thick, is A B C jointed in vertical columns, and locally has a highly scoriaceous upper surface. Above the flow is rhycTite SiOs— _ _------.___.__ 47.54 47.06 52.54 tuff erupted by Mount Taylor. This overlying tuff is Al203______--_-_ ___._. 16.73 7.77 01 OK not baked. At other localities basalt is known to have Fe2O3 ______.___.__ 6. 69 1.30 FeO__- — __ — — .___-__ 6. 67 8. 15 preceded the porphyritic andesite from Mount Taylor, MgO_— __ -.---_..______6.38 13.52 oo but this is the only known locality where basalt preceded CaO--_--_._. _____.._.____ 8.74 19.33 12 34. Na20. __.______-__.__._._ 2.81 .33 3.55 the rhyolite. K20___ -_-__---__._-____ 1. 10 . 11 .42 The basalt sheets on Horace Mesa were erupted after H20-_------_--_. ______.36 .20 . 26 Ti02 ______2. 76 1. 82 Undet. the folding of the McCarty syncline and also after the P206- — ------.51 .06 eruptions of Mount Taylor. But along the west side Cr203 - — - — ---. .--___ Trace MnO___ — ----.__-__ 10 .20 of the mesa a later fault, trending nearly due north, (CoNi)O. ______Trace displaces the lava about 150 feet down to the west. BaO~___ — _-__—-. ------.03 Trace? C02—- — -_ —— -.-___- (?) THE VOLCANIC NECKS 100. 51 199. 85 100. 65 The sheet eruptives originally covered a much greater area than at present. At several localities A. Basalt 6 miles! northeast of Grant. Contains chiefly plagioclase, augite, and olivine, with much magnetite. cited above erosion has exposed natural cross sections B. Augite from A. of the volcanoes, but locally the lava has been com­ C. Feldspar from A. Analysis on 0.75 gram of material. pletely removed from around the volcanic centers leav­ HORACE MESA ing only the resistant core. These cores, or necks, are The erosion surface beneath the sheet eruptives on striking topographic features in the area, particularly Horace Mesa lies at about the same altitude as the in the Rio Puerco Valley northeast of Mount Taylor. surface in Mesa Chivato and La Jara Mesa and was The general aspect of the necks has been discussed by probably originally a connected part of that surface. Button,7 and subsequently Johnson 8 described many Where remote from the Mount Taylor cone the surface of the individual necks in more detail. The prerent slopes about 50 feet to the mile away from the cone, study confirms the essential conclusions of the earlier but the gradient increases toward the cone. The sur­ investigators. face is cut across the outer fringe of tuffs and porphy- Presumably the basalt sheets that cap Mesa Chivato ritic lavas erupted by Mount Taylor, but to the south and Mesa Prieta were once more or less connected on it truncates tilted sedimentary rocks. surfaces that were continuous across what is now the The lavas capping Horace Mesa were erupted from intervening Rio Puerco Valley, and the necks are the four vents of the central type and from one fissure. roots of volcanoes that supplied the sheets. There are Plate 12, A, is a view of one of the cones. The descrip­ about 50 necks of various sizes and dimensions dis­ tions of basaltic lava and cones on La Jara Mesa and tributed through the volcanic field. Most of them are Mesa Chivato apply to Horace Mesa, and only unusual east of Mesa Chivato, but two are west of the mesa and features will be mentioned here. two others south of Mount Taylor. Along the west side of Rinconada Canyon, about Some of the necks are composed mostly of baealt, 2,000 feet south of the experiment station of the New others are mostly basaltic breccia, but most of them are Mexico Agricultural College, is one of the few exposures a mixture of basalt and basaltic breccia. The breccia of basalt erupted from fissures in this field. Here a ' Button, C. E., Mount Taylor and the Zuni Plateau: U. S. Geol. Survey 6th Ann. Kept., pp. 166-179, 1885. 6 Clarke, F. W.. Analyses of rocks and minerals from the laboratory or the United 8 Johnson, D. W., Volcanic necks of the Mount Taylor region, N, Mex.: GeoJ, States Geological Survey, 1880-1908: U. S. Geol. Survey Bull. 419, p. 123, 1910, §90. America Bull., vol. 18, pp. 303-324, 1907, 68 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 is very friable and so easily eroded that necks of breccia CERRO DE IOG CJATES are less conspicuous topographically than the others. The Cerro de los Cuates, 4 miles west of Cabezon The breccia consists of inclusions of sedimentary rock Peak, is an elongate mass of intrusive basalt now rising and basalt in a friable glassy matrix that commonly has about 500 feet above the nearby country. The exposed a distinct flow structure. The basalt inclusions are in main intrusive trends east of north and is estimated to general highly scoriaceous and commonly have flow be about 1,200 feet long and 500 feet wide. The ad­ structure parallel to the flow of the matrix. This joining rocks are Cretaceous sandstone and eandy shale. parallelism of flow structure in separated basalt inclu­ A narrow dike, about 28 inches wide, can be traced sions indicates that they were plastic or liquid masses northward for about 2 miles from a point near the during the formation of the breccia. north base of the neck. Probably the neck is an With few exceptions the country rock adjoining the expanded part of the dike and thus appears to have intrusions was not deformed. At two localities minor been erupted along a fissure. This association of dike tilting of adjacent beds occurred, but there was no and neck is closer than elsewhere in this volcanic field, contact metamorphism except a very slight induration. for most of the necks are of the central type. There is little basaltic breccia at the Cerro de los CABEZOHT PEAK Cuates except along the side contacts, which are best ex­ Cabezon Peak (pi. 13, A) is the highest and most posed on the east side. Jointing at high angles to the impressive of the necks, rising nearly 2,000 feet above side contacts is well developed in the basalt. Toward the Rio Puerco, and the protruding basaltic core is the top of the north peak, however, the joints dip into about 1,500 feet in diameter. The diameter of the pipe the neck, but the significance of this attitude is uncer­ is probably at least 2,000 feet, as indicated by poor tain, because in other necks the intrusion of basalt was exposures of basaltic breccia around the base of the preceded by basaltic breccia, and so this upward flaring basalt core. The country rocks intruded by the neck of the joints may reflect contact with an overlying are Cretaceous sandstone and sjiale that dip gently breccia mass. northward, and although the contact is not exposed Petrographically the basalt at the Cerro de los there is no indication of deformation by the intrusion. Cuates is like that at Cabezon Peak. In the one thin The exposed neck is nearly cylindrical and is about 800 section examined the following percentages of essential feet high. It consists of dense columnar-jointed basalt minerals were estimated: Labradorite, f5 percent; below and highly scoriaceous basalt on top (pi. 13, 5). olivine, 15 percent; augite, 15 percent; magnetite, The joints in the dense basalt immediately below the nearly 5 percent. The feldspar is in small euhedral scoriaceous top are nearly vertical, but downward they laths. The magnetite is finely disseminated, though flare slightly toward the peripheral contact. It is not locally it is concentrated as aggregates of minute certain whether this flaring is due to greater depth, as crystals. Johnson 9 has suggested, or whether the base of the KECK IN GONZA1ES CAWYON neck is closer than the top to the side contacts. The An inconspicuous intrusive mass of basaltic breccia in jointed columns range from a few feet to 8 feet in Gonzales Canyon, about 2 miles west of C^sa Salazar, diameter. Throughout the neck the jointing is much provides some of the most interesting exposures in the more regular than is common in the other necks, volcanic field. (Se:e fig. 11.) The intrusion is nearly probably because of the absence of basaltic breccia circular and about 500 feet in diameter, and no basalt except along the contacts. core is exposed. The exposures are about 1,500 feet Cabezon Peak is slightly higher than the basalt below the erosion surface on which tjie sheet basalts sheets on Mesa Prieta, but these sheets rise gently were erupted. The country rock is sandstone, sep­ northward and if projected would approximately coin­ arated from lower sandstones by 800 feet of shale and cide with the top of the peak. The thick cap of scoria from higher sandstones by 400 feet of sandy shale. on top of the peak is probably a remnant of a cone The basaltic breccia consists of a friable glassy matrix built on the surface of eruption. containing fragments of scoriaceous basalt, and abun­ The basalt at Cabezon Peak consists of about 60 dant xenoliths of sedimentary, rock as much as 3K feet percent of calcic plagioclase (labradorite?), 15 percent in diameter. Most of the xenoliths are sardstone, and of olivine, 20 percent of augite, and 5 percent of mag­ several facts indicate that most of them v^ere derived netite. The feldspar is anhedral, with indistinct from the sandstone adjoining the intrusion: (1) Some borders. The olivine is commonly in considerably of the sandstone xenoliths are abundantly fossiliferous resorbed or altered phenocrysts. The augite is com­ (pi. 14, B}, and the fossils indicate a stratigraphic posi­ monly in small euhedral cyrstals, but the magnetite is tion no lower than the exposed sandstone; (2) sandstone disseminated in slightly larger crystals. xenoliths near the contacts of the intrusion are com­ monly larger, more angular, and more abundant than • Johnson, D. W., Volcanic necks of the Mount Taylor region, N. Mes.: Geol. Soc. America Bull., vol. 18, p. 322, 1907. at the center of the intrusion, suggesting a gradual IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. HEX.

incorporation of the sandstone by the intrusive; (3) the breccia exposed there. Most of the intrusive is basaltic sandstone surrounding the intrusion is highly fractured breccia, but toward the south there is increasingly more for a width of 250 feet, and breccia has been squeezed basalt, culminating in the peak. The basalt in the p^ak into some of the fractures. is jointed in columns that are vertical except near the The disturbed zone around the intrusion consists of a base, where they flare toward the contacts. The top series of very small ringlike (concentrically arranged) of the neck, as at Cabezon Peak, consists of scoria, which faults whose traces are roughly parallel to the sides of is probably a remnant of the cone or crater above the the intrusion. The sandstone is downfaulted, and the neck. faulted blocks are tilted toward the intrusion, producing Sedimentary rocks, are exposed near the south side of a total subsidence of nearly 100 feet. Two narrow the intrusive and are neither baked nor tilted. How­ ring dikes of basaltic breccia were intruded along minor ever, near the north base of the peak there is in the fractures. basalt a shale xenolith thoroughly indurated by baking. The basaltic breccia at this intrusion is entirely Although the metamorphism consisted only of the in­ similar to breccias at the large necks definitely related duration and baldng, this xenolith is in striking contrast to the unaltered country rock along the contact? of these intrusives. The metamorphism of the xenolith is attributed to its position above the basalt. A puzzling exposure in the valley along the west side of La Sefiora Peak shows banded basaltic breccia oT^er- lain by scoriaceous basalt and dipping 25° away from the peak. The contact between this breccia and the country rock is concealed except on the hillside above the valley, where the contact dips 20° into the breccia. Apparently this mass is now completely isolated from the basalt peak. The suggestion is strong that this is a surface flow, but better exposures of rocks in similar relations elsewhere indicate that the mass is intrusive.

SEBOYETA PEAK Seboyeta Peak (pi. 16, .4), 2 miles east of the village of Seboyeta, is an elongate intrusive of andeintic breccia and andesite penetrating shale and tl in- bedded limestone. It trends slightly east of ncHh and is about 1,500 feet long and about 800 feet wide. A nearly vertical contact with the shale is locally exposed along the east side of the neck. Probably the intrusive continues to the north and south as narrow dikes. The neck is composed mostly of breccia intruded by irregular narrow dikes of andesite (pi. 16, B). The breccia consists of a chaotic assemblage of boulders of FIGUEE 11.—Sketch map showing intrusive basaltic breccia in Gonzales Canyon. sedimentary rock and scoriaceous andesite in varying Tbr, Basaltic breccia; Kg, Mesaverde sandstone with strikes and dips indicated; Qal, alluvium. proportions in a glassy matrix. Banding is crude and nearly vertical. to the sheet basalt series and is assumed to be a part of A series of andesite dikes ranging in width from 1 to that series. The intrusion, however, may not have 10 feet intrudes the breccia in the southern half of the penetrated much higher than its present position. neck. These dikes trend at an angle of 45° to the The breccia is interpreted as formed by the alteration long axis of the neck and are spaced fairly regularly at of a roof rock foundering above a rising column of basalt intervals of 50 to 100 feet. In the north half of the (p. 72). neck the dikes are more irregular in trend and vary LA SEWORA PEAK considerably in thickness. There are in addition many The neck referred to by Button and Johnson as Great irregular stringers of andesite in the breccia, probably Neck is locally called La Sefiora Peak. It is an intrusive for the most part offshoots from the dikes. An mass of basalt and basaltic breccia. It is about three- interesting relation was found where one of these dikes quarters of a mile in diameter and may extend north­ intruding breccia is deflected around a boulder of ward for another mile and connect with the basaltic sandstone, 70 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

The andesite at Seboyeta Peak consists roughly of The andesite consists of about 40 percent of oligo- 45 percent of calcic andesine, 10 percent of augite, 15 clase, 10 percent of hypersthene, 35 percent of augite, percent of olivine, and 5 percent of magnetite in an 5 percent of olivine, and 10 percent of magnetite. indeterminate groundmass that probably is mostly OTHER VOLANIC NECKS feldspar. The other necks are also composed of es.rlj basaltic BASALTIC ROCKS BY NEW MEXICO STATE HIGHWAY 6 breccia and later basalt intruded into Upper Cretaceous Where New Mexico State Highway 6 crosses the sandstone and shale. A few of the necks are elongate Rio Puerco there is exposed a conical mass of basaltic and may have been erupted from fissures, but most of bfeccia and basalt south of the highway and a flow them are cylindrical (pis. 12, B; 15, A). There is a of basalt just north of the highway. The contacts of very slight upward drag of strata in contact with the the conical mass are concealed by alluvium. At the neck east of La Abra de los Cerros. At the small edge of the alluvium interbedded basalt and banded andesite intrusions 4 miles east-northeast of La Senora basaltic breccia dip 45°-70° into the cone. The top shale in contact with the andesite is indurated for about of the cone is red, highly scoriaceous basalt. Whether 1 inch from the contact, but at 8 inches from the con­ this cone is truly a lava and cinder cone partly con­ tact no alteration is discernible. Elsewhere the intru­ cealed by alluvium or merely a neck left protruding sions neither tilted nor altered the adjoining country by erosion of the surrounding country cannot be fully rock. A contributing reason for the general absence of determined. The basaltic flow with no other known metamorphism, however, is probably the common oc­ nearby source suggests that the conical mass is a lava currence of breccia between the basalt and the country and cinder cone. However, masses similar to this rock. cone and known to be necks are exposed in other The basaltic breccia consists of xenoliths, some of parts of the Mount Taylor field. them many feet in diameter, of sedimentary rocks and basalt. Most of the basalt inclusions are highly scori­ The basaltic flow north of the highway overlies aceous, but some are dense. The fragments of sedi­ part of the Santa Fe formation and is overlain by higher mentary rock are generally well rounded and enclosed beds of that formation (pi. 19, J5). At the largest by a zone of alteration. The matrix of tH breccia is exposure adjacent to the highway the lava is tilted friable glass, commonly with distinct flovT structure. above 8° S., and a small fault displaces the north edge Much of the breccia shows banding, which is at least of the exposure. Other exposures to the northeast partly due to varying proportions of sedimentary and show that the dip of the lava changes to east in con­ basalt inclusions. The bands are commonly tilted but. formity with the change in dip of the Santa Fe beds. not folded. Isolated inclusions of basalt as a rule show The flow is much lower than the sheet basalts nearer a flow structure mutually parallel and parallel to the Mount Taylor, but this may be due partly to down- banding. At Santa Rosa Peak, as at La P^ilora Peak faulting (p. 77). The flow is probably either contem­ (p. 69), the banded breccia dips at fairly steep angles poraneous with or younger than the sheet basalts around away from the neck. More commonly, however, the Mount Taylor. SIERRA ALESNA breccia along a contact dips toward the neck, generally between 10° and 40° (pis. 14, A; 17, J5). Breccia is The Sierra Alesna lies northwest of Mount Taylor, generally present along the contacts, but irregular about 10 miles north of San Mateo. The neck intrudes masses of breccia also occur throughout some of the sandstone and shale and rises 1,200 feet above the sur­ necks. rounding area. The country rock is well exposed on Many of the necks have cores of basalt, and this the north side, about 600 feet below the top of the neck, basalt was intruded after the breccia rras formed. and though within 175 feet of the neck is neither tilted Some of these cores contain little or no breccia, as at nor baked. The intrusive mass is nearly circular and Evans Peak (pi. 15, A, I?); others contain abundant therefore of the central type, but several outliers of breccia—for example, the neck west of La Abra de los andesite south of the neck are undoubtedly connected Cerros and Cerro Chato. Still other necks contain with it by a dike trending slightly east of north. intermediate proportions of breccia and basalt. Wher­ The neck is composed entirely of dense andesite ever breccia is scarce the basalt is jointed in fairly jointed in nearly vertical columns. Near the base the regular columns, and near the contacts the columns are joints dip about 70° toward the contact, but they curve parallel to platy joints that are developed along the upward and at the top dip about 80°. These joints contact. At these localities, therefore, cooling of the produce the structural control that has given the neck basalt progressed downward from the crater rather its awl shape, from which the Spanish name for the than inward from the contacts. Where breccia is as­ peak was taken. Around the base of the neck the sociated with the basalt there is generall^ a zone of joints are a little over 1 foot in diameter. They are platy-jointed basalt next to .the breccia, and the columns four- to six-sided, though most have only four sides. show a strong tendency to curve toward the plates. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 14

A. VOLCANIC NECK EAST OF LA ABRA DE LOS CERROS. B. XENOLITH OF FOSSILIKEROUS CRETACEOUS SANDSTONE View looking east. The country rock is Cretaceous"shale (Ku). There IN BASALTIC BRECCIA IN NECK IN GONZALES CANYON. is a vertical contact between the shale and the banded basaltic breccia (Tbr) that dips into the neck. Irregular masses of andesite or basalt (Tan) with columnar jointing intrude the breccia. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 15

*

A. VIEW FROM THE EAST. The neck is composed of andesite (possibly basalt) and protrudes about 1,100 feet above the level of the foreground. Photograph by D. A. Andrews.

B DETAIL OF COLUMNAR JOINTING AND OUTER SHELL OF IRREGULAR PLATY JOINTING. Photograph by R. C. Hecker. VIEWS OF EVANS PEAK. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 16

v4. VIEW LOOKING SOUTHWEST TOWARD SEBOYETA PEAK. The peak is composed mostly of andesitic breccia intruded by andesite dikes, some of which can be seen at the left protruding above the lower part of the neck.

B. XENOLITHS OF SEDIMENTARY ROCK IN BRECCIA ALONG EAST SIDE OF SEBOYETA PEAK. Andesite dikes, protruding as dark masses, can be seen at the upper left. GEOLOGICAL SURVEY ' PROFESSIONAL PAPER 189 PLATE 17

A. DIKE OF BASALT AND BASALTIC BRECCIA INTRUDING C. ANGULAR UNCONFORMITY BETWEEN SANTA FE FORMATION AND UNDERLYING CRETACEOUS CRETACEOUS SHALE. ROCKS. In sec. 3, T. 14 N., R. 3 W. The Santa Fe (Tsf) dips 10° S. 20° W.; the Cretaceous rocks (Ku) dip 18° S. 50° E. View looking south in center of sec. 3, T. 14 N., R. 1 E.

B BANDED BASALTIC BRECCIA LYING AT OUTER CONTACT D. ANGULAR UNCONFORMITY BETWEEN SANTA FE FORMATION AND UNDERLYING CRETACEOUS OF A LARGE BRECCIA MASS. ROCKS. The banded breccia dips into the mass. In sec. 19, T. 14 N., R. 4 W. The Santa Fe (Tsf) dips 7° S.; the Cretaceous rocks (Ku) dip 30° S. View looking east in center of sec. 4, T. 14 N., R. 1 E. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 71

These relations occur around xenolithic masses of and can be traced almost continuously for several hun­ breccia as well as against the breccia along the contacts dred feet. It is nowhere more than 2 feet in tl ickness (fig. 12). and generally about 18 inches. Locally the dike is Even in the very small intrusive 4 miles east-north­ composed wholly of basalt horizontally jointed in small east of La Sefiora Peak there is evidence of consider­ columns normal to the side contacts. Elsewhere basal­ able loss of heat upward. The rock is andesite jointed tic breccia is associated with the basalt, occurring

N. S. along one side of the dike or more commonly in the middle. Plate 17, A, shows an outcrop where basaltic breccia separates two basalt portions of the dike, a relation that persists for a considerable distance along the outcrop. This outcrop is particularly interesting in connection with the necks, because the breccia, like that in the necks, contains small xenoliths of sedi­ mentary rocks and highly scoriaceous basalt in «, friable glass matrix. On each side of the breccia there is basalt 6 to 8 inches wide jointed in horizontal columns. The basalt in contact with the shale is dense and non- scoriaceous, but there are vesicles along its contact with breccia, strongly suggesting that the breccia al­ 25feet lowed release of pressure existing in the fluid basalt. Ku This exposure is about 1,500 feet below the erosion FIGUBE 12.—Diagrammatic section through north side of Juan Tafoya Peak. Ku, surface on which the sheet basalts were eruptec1 . The Cretaceous shale and sandstone; Tt>r, basaltic breccia; Te, basalt. The trend of lithology of this dike suggests that it is related to the columnar joints in the basalt is indicated by the broken lines. sheet basalts and to the necks, but it may have been in columns that in plan radiate from the center of intruded during the later period of minor faulting that the mass (fig. 13). In cross section, however, the occurred in this part of the volcanic field wl en the columns are horizontal against the outer contact with drainage system was between 100 Cretaceous shale, but inward the columns curve upward and 900 feet above its present and at the center are nearly vertical. At Picacho Peak, position (p. 75). The basalt in southeast of Mount Taylor, the joint relations provide this dike consists chiefly of calcic a notable exception to these general descriptions. The andesine with some augite and intrusive mass is elongate and consists wholly of dense traces of magnetite in a glassy andesite jointed in columns that are horizontal and groundmass. normal to the side contacts. A basalt dike trending east of The basalt in most of the necks contains 60 to 70 north crops out at intervals near or percent of sodic labradorite or calcic andesine, 15 to along a fault about 3 miles west- 25 percent of augite, 5 to 15 percent of olivine, and northwest of Casa Salazar. If the magnetite. The olivine occurs commonly in large dike is continuous, as seems likely, ^ phenocrysts: one that was collected is nearly 1 inch its thickness ranges from about 18 u" , i n f i TI • i FIGURE 13.—Diagram of co- long. Where basalt solidified against breccia con­ inches to 6 teet. It is composed ]umnar joilltjng in intru- siderable glass developed, but where basalt solidified mostly of basalt crudely jointed sions 4 miles east-northeast of La Sefiora Peak. A, Plan against basalt there is little glass. in horizontal columns. At one of radial jointing*; B, East- locality a dikelike offshoot extends west cross sectioi showing BASALT DIKES joints curving upward near a few hundred feet to the west. center of intrusion. Numerous basalt dikes, probably related to the ba­ Where it joins the main dike there saltic sheet eruptions, are exposed in the Mount Taylor is considerable breccia, and the associated basalt is very volcanic field. All trend north, conforming to the irregularly jointed. regional strike of the faults, but only one (p. 72) is At Paguate Indian pueblo a dike crops out ttat can known to have been intruded along a fault surface. be traced southward for about 2 miles and northward The dikes are most abundant east of Mount Taylor for about 1 mile. The dike ranges from 2 to 12 feet in and Mesa Chivato, but a few are locally exposed south width and intrudes thick sandstones and inter! ^dded of the mountain. The dikes are thin, most of them shale. It is mostly breccia, consisting almost wholly about 2 feet thick, but some attain a maximum of of friable glass containing small fragments of sedi­ about 30 feet. mentary and igneous rock, with locally a ver^ thin On the north line of sec. 2, T. 14 N., R. 4 W., about border zone of basalt. The dike is more easily eroded 3 miles west of Casa Salazar, a nearly vertical dike than the enclosing sandstones and forms a depression trends slightly east of north. The dike intrudes shale where it cuts them. It is about as susceptible to ero- 72 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 sion as the soft shale, and where shale is the country should be preserved with the lavas. There is prac­ rock exposures of the dike are few except in arroyos. tically none. Thin beds of light-colored tuff locally No evidence of alteration of the country rock was interbedded with the basalt sheets bear no resemblance observed anywhere along the dike. to the basaltic breccia. Locally where there are natural Only one dike is known in the severely faulted portion cross sections of the volcanoes, as in Marquez Canyon, of the area. The outcrop, a mile southeast of the Ojo large masses of breccia are preserved along tl Q, walls of Escondido fault, 4% miles east of Suwanee, was found the pipe, but little or no breccia occurs with the over­ by G. F. Taylor. The dike trends about N. 25°-30° E. lying flows. Some basaltic flow breccia exposed in the and dips 65° NW. The trend closely parallels that of Grant Eidges (pi. 11, A) differs from the breccia in the the many nearby faults, but no displacement was necks by the comparative absence of a cementing glass found along the dike. The rock is finely crystalline matrix and very low proportion of sedimertary rock andesite composed mostly of calcic andesine, consider­ fragments. It consists almost wholly of boulders of able augite, a little magnetite, and traces of olivine. frothy basalt. Another dike crops out about 1 mile northeast of 3. As the breccia is lighter than the basalt the breccia McCarty, south of Mount Taylor, and there are masses, if pyroclastic, must have fallen into open con­ several others near Chico Arroyo, north of Mesa duits. But some of the conduits are ver^ narrow Chivato. Most of these dikes parallel the trend of fissures many hundreds of feet deep in poorly consoli­ nearby faults, but, so far as known, none of them follow dated shale (p. 71; pi. 17, A). It is unlikely that they fault surfaces. could remain open to collect debris from tie surface. At the mouth of Einconada Canyon, south of Mount 4. At the breccia neck in Gonzales Canyon most of Taylor, H. R. Joes ting and R. C. Becker found Cre­ the fragments are sandstone, in part fossiliferous and taceous sandstone and shale displaced 150 feet by a surely derived from the adjacent sandstone formation fault along which a basaltic dike was intruded. The (p. 78). These inclusions are practically at their original dike is 30 feet wide and closely follows changes in location. Furthermore, they grade from large angular trend of the fault. Toward the south end of the ex­ blocks near the side contacts to fine particles in the posure the dike widens considerably, and higher vol­ interior of the intrusion. canic rocks funnel out over tuffs, about 70 feet thick, These relations pre more consistent with an assump­ that probably belong to the rhyolite tuff series erupted tion that the breccia is of nonextrusive origin. But the by Mount Taylor. The higher volcanic rocks may be rising columns of basalt did not fold or fault the adjacent a remnant of an elongate cone built by a fissure erup­ country rock and produced practically no drag: along the tion. Both the fault and the dike die out beneath the side contacts, so the breccia is attributed to progressive cone and do not extend farther south. This is the only alteration and incorporation of roof rock by the rising known locality in the Mount Taylor volcanic field intrusions. Each of the strata is visualized as being where a dike follows a fault fissure. The occurrence progressively more altered and deformed as the intru­ is also unusual for this field in that the basalt is rela­ sions rose, and at any given time there was complete tively very coarsely crystalline and could be described gradation from undisturbed roof rock through breccia as dolerite. It is composed of sodic labradorite, augite, to relatively unadulterated basalt. and minor quantities of olivine and magnetite. Whatever hypothesis is proposed must confine the disturbing effects of the intrusions to a pipelike zone ORIGIN OF THE BRECCIA through. the roof rock, and the subsequent intrusion It seems probable that the breccia is a nonextruded must occupy all of that pipe, because the sedimentary byproduct of the intrusive process. My reasoning is rocks beside the intrusions are not disturbed. Probably not that certain facts indicate an intrusive process but the pipe of the deformed strata was a long slender cone, that those facts are difficult to explain by a theory of and in each stratum the firpt eflect of thy intrusion was pyroclastic origin. The evidence may be summarized at a point—the apex of the cone. As intrusion con­ as follows: tinued the cone of disturbance was extended upward, 1. Under a theory of pyroclastic origin each breccia and in each of the strata the disturbed area was mass must represent several falls of fragmental material. extended laterally. But the material now preserved could not have fallen Probably the first effect was a baking and induration. back into the vents after each explosion, for the bands This would culminate in abundant jointing and, in are a conformable series. Each breccia mass must water-bearing sandstone, further fracturing by steam have fallen back as a large unit block (pi. 17, B), a land­ explosions. A push upward or a downward settling at slip from the cone. But none of the preserved cones this stage would develop minor faults across and ring are composed of such material. faults around the fractured zone, contributing to further 2. If the tremendous quantity of breccia now pre­ break-up of the roof. The upper part of the laiTa column, served in the necks represents pyroclastic material probably more frothy and richer in volatile constituents that fell back into the vents some pryoclastic material than the main column, could be intruded alon^ the frac- IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 73

ture planes, and the addition of this material would con­ the Zuni Mountains, others are north of the villlage qf vert the breccia to a plastic mass. Considerable churn­ Bluewater, and others are near Laguna Pueblo. The ing of the plastic breccia would comminute the frag­ flows have exceedingly fresh surfaces, which, combined ments of sedimentary rock and intimately mix the fine with their position in the valley bottom, make them particles with the enclosing medium. Irregular stringers appear as if they had been flowing only yesterday. of basalt intruded at this stage would be churned in These flows are more ferromagnesian than most of the with the breccia and form isolated and relatively pure sheet eruptives of the Mount Taylor volcanic field. masses of basalt in the breccia. Rise of the basalt They consist of labradorite with a high percentage of column through this mass would squeeze the breccia olivine, slightly less augite, and some magnetite. against the side contacts and there develop the banding STRUCTURAL GEOLOGY and flow lines. This hypothesis does not satisfactorily explain why GENERAL STRUCTURAL RELATIONS OF THE VOLCANIC FIELD the banding and flow lines of the breccia masses so Most of the Mount Taylor volcanic field is in the commonly dip into the intrusions. But to assume that Colorado Plateaus province, but the field extends east­ the masses were rotated to this attitude by rise of the ward into the Basin and Range province. (See fig. 3.) adjacent basalt column is as reasonable as to assume The Colorado Plateaus are represented in northwest­ that they descended from the surface into open conduits, ern New Mexico by the San Juan Basin, a broad struc­ as required by the theory of pyroclastic origin. tural depression. In the interior of the basin the beds ^Basalt in the breccia is commonly highly scoriaceous, are nearly flat, but on all sides they rise gently to the and this is the chief basis for the pyroclastic hypothesis. bordering mountain uplifts. The north flank of the However, the superior jointing of basalt in contact with basin, in southwestern Colorado, is a steep monoclinal breccia indicates that the breccia was more potent in flexure produced by the uplift of the San Juan Moun­ promoting consolidation than the sandstone or shale of tains. A monocline forming the west flank of the basin the country rock. Furthermore, basalt in contact with is produced by the Defiance uplift and by a laccolithic breccia commonly developed vesicles, but basalt of the injection at Carrizo Mountain. The east flank of the same intrusion in contact with sedimentary rocks is basin is in part a steep monocline along the Nacimiento dense. Apparently, therefore, the breccia allowed re­ Mountains, a range of the southern Rocky Mountains, lief of pressures and release of any gases that may have at the northeast corner of the Mount Taylor volcanic been contained in adjacent basalt, and this facilitated field. From the base of the monocline westward the consolidation with development of vesicles. beds dip gently toward the interior of the basin. The Nacimiento Mountains consist of a central core of SUMMARY OF BASALT ERUPTIONS basement rocks uplifted in an asymmetric anticline, with local reverse faulting along the steep west flank. Sheets of basalt were erupted from a large number of On the south side alone the basin is not bordered by a vents distributed around Mount Taylor. One local monoclinal fold. A gentle northward dip prevail? there, basalt eruption preceded the rhyolitic tuff, and a few produced by the upliet of the Zuni Mountains and Mesa basalt flows preceded the later flows of the porphyritic Lucero. The Zuni, Lucero, and Nacimiento uplifts andesite series, but the greatest volume of basalt was may have been more or less contemporary in the inter­ erupted shortly after the Mount Taylor volcano had val between early Eocene and late Miocene time.10 become quiescent. Most of the flows spread out in The south flank of the San Juan Basin, on which sheets on erosion surfaces developed around the Mount Mount Taylor is located, departs considerably from a Taylor cone and locally overlapped the sides of the simple basinward dip. West of the Zuni Mountains is cone. The sheets are preserved today capping high the Gallup-Zuni Basin, a northward-plunging asym­ mesas that extend as foothills away from the Mount metric syncline. Immediately east of the Zuili Moun­ Taylor cone. Most of the sheets were supplied by tains is the northward-plunging McCarty syncline, eruptions of the central type, though a few fissure a broad, shallow, symmetrical fold. The McCarty eruptions are known. Apparently the basalt made its syncline extends northward into the south flank of the way through the upper part of the crust by some process San Juan Basin and plunges steeply under the Mount other than forcible injection, because even where the Taylor volcano. The area shown on plate 7 therefore country rock is soft shale the eruptions caused no includes the northern extension of the McCarty S7ncliiie deformation. and parts of the north flanks of the uplifts on each side RECENT FLOWS of the syncline. Very recent flows of basalt are found in the valley The central and western parts of the Mount Taylor bottom of the Rio San Jose along the south border of volcanic field, limited eastward by the Alamosa fault, the volcanic field. These flows were given only casual San Ignacio monocline, and Reservoir fault, are in the attention, because none were supplied by vents within 10 Hunt, C. B., Tertiary structural history of part of northwestern New Mexico the field. Some of the vents are along the east side of [abstract]: Washington Acad. Sci. Jour., vol. 24, pp. 188-189,1934. 74 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

San Juan Basin and constitute the portion of the abrupt northward plunge. Under Mount Taylor the volcanic field in the Colorado Plateaus. structural relief for 10 miles on each si'de of the axis is The southwest quarter of New Mexico is in the Basin at least 1,500 feet. North of Mount Taylor the north­ and Range province. It is an area of abundant faulting ward plunge of the syncline is considerably less than and more or less folding. In the southern part of the the regional dip, and near the north border of the State, west of the Rio Grande Valley, there are over- volcanic field the syncline dies out into the regional thrusts combined with folds and intersecting sets of dip. Near Mount Taylor the syncline is strongly normal faults. Along the valley, however, there are asymmetric, the west flank being a steeply dipping only broad northward-trending folds broken by normal monocline. This monocline extends from the south strike faults that produce horsts and grabens. The side of T. 12 N., R. 9 W., to the north side of T. 14 N., folded and faulted belt of the Rio Grande Valley extends R. 8 W. Many dip faults, mostly small, fracture the northward across the eastern part of the Mount Taylor northern part of the monocline. Two miles west of volcanic field to the south tip of the Nacimiento San Mateo there is a strike fault with a downthrow of Mountains. about 700 feet to the east, which apparently takes up In the Mount Taylor volcanic field, east of the most of the monoclinal displacement and dies out under Alamosa fault, San Ignacio monocline, and Reservoir La Jara Mesa. South of this mesa a second monocline fault, the block-faulted Cretaceous strata are exposed, produces about 1,600 feet of structural relief in about but farther east they are concealed by the fluviatile 1 mile. This monocline (fig. 5) is probably faulted, Santa Fe formation. This part of the volcanic field is but the outcrops are so few that detailed recorstruction the east edge of a graben that extends 25 miles eastward of it was not attempted. past Albuquerque to Sandia Mountain. The Santa Fe Apparently the east flank of the McCarty syncline formation, deposited in the topographic depression that in the Mount Taylor field is fairly regular, and it dips resulted from the graben faulting, thickens rapidly westward at about 200 feet to the mile. eastward. The part of the Mount Taylor volcanic The sheets of basalt that followed the eruptions of field lying in the graben constitutes the Basin and Mount Taylor lie on erosion surfaces cut across the Range portion of the field. edges of strata dipping into the deepest part of the The structural deformation of the Mount Taylor McCarty syncline. On La Jara Mesa the erosion volcanic field began probably in late Miocene time and surface and overlying eruptives truncate the nonocline continued through the Pliocene (p. 56, fig. 8). In the at the south side of the mesa (fig. 5) and the strike Colorado Plateau portion of the field the movements fault at the north side. Erosion surfaces and overlying began with the uplift of the Zuni Mountains and Mesa sheet basalts truncate the northward-plunging syncline Lucero, which produced the regional northward dip along the south side of San Fidel Mesa. Similar and the folding of the McCarty syncline. The date of truncation of the east flank of the syncline is seen in the folding and faulting northwest of the syncline and the canyons that have been eroded into the east side the gentle warping just east of it is uncertain, but they of Mesa Chivato. were probably contemporaneous with these early movements. This deformation preceded the eruptions MINOR FOLBS WITHIN McCARTY SYNCLINE of Mount Taylor. Minor faulting east of the McCarty syncline and minor folding and faulting within the There are several minor folds south of Mount Taylor syncline occurred much later, after the eruption of the in the McCarty syncline. At the head of Einconada post-Mount Taylor sheets of basalt. In the Basin Canyon, in the eastern part of T. 11 N., R. P W., is a and Range portion of the field deformation was nearly dome covering about \% square miles and having a completed before eruption of the sheet basalts and structural relief of about 1,000 feet (pi. 7). deposition of the Santa Fe formation, but some move­ The attitude of the Mesaverde beds at the head of the ments continued until after the eruptions, while the north fork of Lobo Canyon, in the southern part of Santa Fe was being deposited. T. 12 N., R. 8 W., indicates another closed fold. Because of the uncertain position of the exposed rocks COLORADO PLATEAU PORTION OF THE FIELD in the Mesaverde formation, this fold was not contoured. McCARTY SYNCLINE Probably, however, the structural relief of this fold is The largest fold in the Mount Taylor volcanic field not as great as that of the fold at the head of E,inconada is the McCarty syncline, the axis of which trends about Canyon. N. 20° E. across the central part of the field. (See The location of these two folds at the heads of the pi. 7 and fig. 3.) South of this field the syncline is two largest canyons draining the southwest, side of shallow and symmetrical, with flanks rising gently to Mount Tayior is puzzling. No direct evidence was the bordering uplifts, the structural relief for 10 miles found indicating that the two folds are later than the on each side of the axis being only about 600 feet. At eruptives capping the canyon rims. Although the the south border of the Mount Taylor field there is an outcrops of the flows nearly parallel the strike of flanks IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 75 of the folds, these outcrops are so far from the area of are found over much of the south flank of the basin. steepest dips that no disturbance of the eruptives was The faults are crowded in the areas of steepest folding. noted. It would seem reasonable to interpret these To the west and northwest, concomitant with the di­ domes as later than the lavas, including the sheet minishing intensity of folding, the faults not only be­ basalts, and also to regard the canyons as having been come progressively fewer but produce less displacement. eroded because of the disruption of the resistant lava The close relationship between degree of folding and cap. But the basalts overly ing the dome in Canyon degree of faulting strongly suggests an intimate genetic Seco, near the south line of T. 11 N., R. 7 W., are not relationship between the two types of deformation. It folded and thereby discount that interpretation. seems likely that they were contemporaneous, but it is A remarkable feature of the dome in the southeastern impossible to date the movement closely. The young­ part of T. 11 N., R. 8 W., is the abrupt flexure of the est beds involved are of lower Pierre age. Probably, west flank. There is no intermediate dip between the however, the movements were much later than early flat beds to the west and the nearly vertical beds of the Pierre time and were related to the folding of the west flank. This abrupt change in dip and the fact McCarty syncline, the most southeasterly and largest that these domes produce considerable structural relief of the folds, regarded as later than early Eocene and within very small areas suggest forces of igneous earlier than late Miocene (p. 57, fig. 8). injection, but no evidence was found of alteration or baking of beds on the crests of the domes. GENTLE WARPING EAST OF McCARTY SYNCLINE Folded sedimentary rocks belonging to the Mesa- East of the McCarty syncline the Cretaceou^1 rocks verde formation are exposed in the bottom of the are very gently folded. These rocks rise toward the Mount Taylor amphitheater near the east side. The east and southeast, where the undulations are termi­ easternmost beds of this exposure dip 20° E. They nated abruptly by faults and a steep eastward-dipping are overlain by not less than 400 feet of rhyolitic tuff. monocline. This border of gentle folding lies along the The tuff also dips to the east, but the dip ranges from Alamosa fault, San Ignacio monocline, and reservoir 10° to 20° and may be depositional rather than struc­ fault, at the southeast rim of the San Juan Basin. tural. The source of the tuff was the crater of the Sheet basalts lie on erosion surfaces that truncate Mount Taylor volcano, not more than 2 miles to the the northward-dipping and gently undulating strata west, and the tuff may have accumulated on dip slopes. east of the McCarty syncline. The warping is accord­ No exposures of tuff were found associated with the ingly interpreted as probably contemporaneous with steeper northward-dipping beds of the northernmost the folding of the syncline. part of this Mesaverde exposure (p. 58). The faults in the north half of this portion of the field That some of the faulting within the McCarty are paralleled by several basaltic dikes. There is, how­ syncline was later than the formation of the high erosion ever, no alinement of the necks, and apparently none surfaces and of the sheet basalt is indicated by the of these intrusions followed the faults as conduits to relations along the southwest side of Horace Mesa, the surface. The faults seeni related to the blocV faults where a fault trending slightly west of north drops the in the Basin and Range part of the field, for ttey cer­ erosion surface and overlying basalt 150 feet on the west. tainly show a marked parallelism of trends.

FOLDS NORTHWEST OF McCARTY SYNCLINE Little evidence is available to limit the date of the faulting in this gently warped part of the area. Along Northwest of the McCarty syncline the prevailing the north side of Chico Arroyo, 5% miles west of the northward dip is interrupted by several folds. In the Cerro de los Cuates, Cretaceous rocks are displaced 50 Mount Taylor volcanic field the folds are large and feet by a fault. However, gravel about 70 feet above prominent structural features, as shown by the struc­ the flood plain and 100 feet above the arroyo lies un­ ture contours on plate 7. Westward, however, folding disturbed across the fault. The final movements on progressively diminishes, and near the west border of this fault had therefore taken place prior to tl n, time the adjoining area, mapped by Sears,11 only broad and when the stream .flowed 100 feet above its present po­ very gentle undulations interrupt the regional dip. sition. If this faulting is contemporaneous with the There is a similar diminution of intensity of folding to fault along the southwest side of Horace Mesa the the northwest.12 The folds in the western part of the faulting occurred while the drainage system vas be­ Mount Taylor volcanic field therefore represent a maxi­ tween 100 and 900 feet above its present position. mum of the deformation and are not representative of The part of the area between Mesa Chivato and Mesa the south flank of the San Juan Basin. Prieta was the scene of extensive volcanism when the The two sets of faults that are present in the Mount sheets of basalt were erupted. These volcanic flows Taylor volcanic field northwest of the McCarty syncline thus rose through the gently warped series of rooks and « Sears, 3. D., The coal field from Gallup eastward toward Mount Taylor, N. Mex.: not through the folded and faulted area northwest of U. S. Geol. Survey Bull. 860-A, pi. 1, 1934. the McCarty syncline or the severely faultec1 Basin is Hunt, C. B., and Dane, C. H., Preliminary map showing geologic structure of the southern part of the San Juan Basin, N. Mex., U. S. Geol. Survey, 1933. and Range part of the volcanic field. 76 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937

BASIN AND RANGE PORTION OF THE FIELD FAULTS

GENERAL STRUCTURAL RELATIONS The Ojo Escondido fault, in the Antonio Sedillo grant, pursues a sinuous course across an are??, of little The most striking structural feature of the Mount topographic relief. It is a normal fault, with the west Taylor volcanic field is the sharp boundary between the block dropped. The displacement increases northward Basin and Range and Colorado Plateau portions of the to about 3,500 feet where the fault passes under cover field. The boundary is marked by the Alamosa fault, of the Santa Fe formation. Locally along the fault San Ignacio monocline, and Reservoir fault (pi. 7), there is a shattered zone, as much as 300 feet wide. which have dropped the much-faulted and steeply tilted The material in this zone ranges in size from finely rocks to the east with respect to the gently warped crushed rock to boulders measurable in scores of feet. rocks of the plateau. There is practically no transition Where the zone thins out and the fracture is a single zone between the areas showing the two types of clean break the fracture surface dips 70°-45° W. deformation. The Suwanee fault, about 2% miles west of the Ojo The trace of the Alamosa fault was not found, but its Escondido fault, also has considerable stratigraphic dis­ position locally can be determined within a few hundred placement down to the west. Where it emerges from feet. The displacement is between 2,000 and 2,500 alluvial cover at the south, the trend of the fault is feet (pi. 18, A). The fault dies out toward the north, N. 10° E. and the displacement is 500 feet. A mile to but the displacement is maintained by the faulted San the north the fault trends N. 20° E., and where the Ignacio monocline, which has an average dip of about fault passes under cover at the north the displacement 40° E. (pi. 18, B). The monocline in turn dies out is increased to nearly 1,200 feet. northward in a series of small faulted folds (pi. 7). East of San Ignacio there are nine nearly parallel Where the San Ignacio monocline dies out an arbitrary faults, in general producing steps down to the west. boundary line between the two provinces must be drawn The most westerly of this group, the Ojito fault, has a eastward to the Reservoir fault, which, like the other stratigraphic displacement of 500 feet and dip^1 45°-50° boundary faults, drops the rocks in the Basin and W. Most of the other faults dip between 70° and 85°. Range province with respect to the Colorado Plateau The San Fernando fault is traceable for onH a short part of the volcanic field. The Reservoir fault dies out distance in T. 12 N., R. 1 W. It is worthy of special northward in the southward-plunging folds produced mention because with the exception of the boundary by the Nacimiento uplift. faults it is the one fault known to have corsiderable Several hundred faults are exposed in the Basin and stratigraphic displacement down on the east side. The Range portion of the Mount Taylor volcanic field. amount of the displacement is uncertain, although it is Most of these are small, but some produce displace­ probably near 1,500 feet. Locally along the fault there ments of a few thousand feet. So far as known all the are zones of shatter 200 feet wide. Southward the faults are normal, and the displacement is generally fault branches into several minor faults. To the north down to the west. The faults commonly dip west at it passes under cover. only moderate angles. The Ojo Escondido fault, for The Garcia fault, concealed except in sees. 30 and 31, example, at one locality produces a displacement of T. 14 N., R. 1 E., has a minimum stratigrs.phic dis­ 3,000 feet, and the fault dips only 45° W. Several of placement of 2,200 feet down to the west. The true the smaller faults, such as the Ojito fault, have equally figure is probably nearer 3,000 feet. The fault is poorly moderate west dips. exposed and passes under the cover of the Santa Fe Evidently the fault surfaces are very irregular. The formation both to the north and to the soutl. dips of some faults change as much as 35° in short distances along the strike. Other faults have sinuous FOUDS traces across areas of low relief. Striae on the fault The southern part of the block that is faulted down by surfaces can rarely be found, because the faulted rocks the Ojo Escondido fault has been folded into r, syncline are very friable. The few striae observed are at high whose axis trends about N. 45° E. and plunges north­ angles. These relations suggest that'movement on the eastward into the fault. faults was chiefly vertical. In the southeastern part of T. 12 N., R. 3 W., there There is a striking parallelism of the fault traces, the is a northward-trending syncline a short distance east vast majority of which trend N. 10°-15° E. The of the San Ignacio monocline. apparent fault dips show less parallelism. At the north edge of the Basin and Range p^,rt of the Very few of the faulted blocks have been folded. In volcanic field two synclines and an anticline plunge general there is a fairly uniform east clip within any southward. These are exceedingly sharp folds, but single block, but the amount of dip in different blocks they are within the area deformed by the Nacimiento is highly variable. uplift and can be regarded as features related to that The following descriptions are confined to only a few uplift, so that similar folds are not to be expocted far­ of the most striking structural features. ther south in the area of block faulting. IGNEOUS GEOLOGY AND STRUCTURE OF MOUNT TAYLOR VOLCANIC FIELD, N. MEX. 77

AGE OF THE DEFORMATION basalts were roughly contemporaneous with the overlap The structural movements in the Basin and Range of the Santa Fe into the Basin and Range part of the part of the Mount Taylor volcanic field can be closely Mount Taylor volcanic field, and that the basalts were dated with respect to the Santa Fe formation, generally erupted after the major fault movements but were regarded as of late Miocene and Pliocene age, which in involved in the latest deformation. The basalts and this field is overlain by deposits of probable Pleistocene underlying erosion surfaces extend eastward nearly to age (p. 56). The major movements of faulting had the edge of the plateau, as at Mesa Lucero, south of the clearly preceded Santa Fe deposition in the eastern part field, and at Mesa Prieta. Figure 15 illustrate? these of the volcanic field, but the latest movements involved relations. Evidently most of the movement along the the lower part of the formation. There is invariably a fault shown in the figure preceded the cutting of the distinct angular unconformity between the Santa Fe erosion surface beneath the sheet basalt. If it were and the Cretaceous beds, and the latter dip much more steeply. Rarely is there a pro­ Lucero Mesa nounced difference in strike. Faults displac­ Sheet basaltCPIiocene?) ing the Santa Fe beds almost invariably displace the Cretaceous by considerably greater amounts. Pl'ocene?', These relations are well exposed through­ out the northeastern part of the field. For example, in sec. 4, T. 14 N., R. 1 E., the FIGCJKE 15.—Diagrammatic section illustrating faulting east of Mesa Lueero. After Darfton. The basalt and the erosion surface on which it lies are probably younger than the fault at the right. Cretaceous has been tilted 30° S., but the over­ (See also fig. 8, p. 57.) lying base of the Santa Fe has been tilted only 7° in the same direction (pi. 17, Z>). A mile east assumed that the faulting followed the cutting of the of this locality the Cretaceous beds strike N. 40° E. erosion surface, then a reconstruction of the pr3-fault and dip 18° SE., whereas the overlying Santa Fe strikes relations would show Cretaceous rocks capping a west­ N. 70° W. and dips 10° SW. (pi. 17, C}. ward-facing scarp 2,000 feet higher than the erosion The best example of the relation of the Santa Fe to surface. This scarp would have been about 50 miles the faulting is found just outside the mapped area, a long, and the assumed later faulting would have coin­ few hundred feet north of State Highway 44, about half cided with it. This assumption of the fault coinciding a mile west of San Ysidro. Here Upper Cretaceous with an extensive 2,000-foot scarp is improbable. beds have been faulted down about 2,500 feet on the Other evidence, however, cited below, indicates that east against Triassic or Permian red shale, whereas the the basalt did not entirely escape the fault movements. Santa Fe formation and the erosion surface beneath it 1. One tilted and faulted basalt flow, interbedded with have been displaced only 7 feet by the fault (fig. 14). the lower part of the Santa Fe, is known in the Basin and Range part of the volcanic field (p. 70). 2. The erosion surface beneath the Santa Fe at the fault locality near Highway 44, half a mile west of San Ysidro slopes away from Sierrita Mesa, in the Nacimiento Mountains. The surface is about f0 feet above the flood plain of Arroyo Salado and about 100 feet above the stream bed and is only 2 to 6 mile^1 from the nuclear area. Certainly this surface is younger than the basalt-capped surface on Mesa Prieta, wlich is about 900 feet above the present drainage level and 20 to 30 miles from any known nuclear area. The lower FIGUBE 14.—Sketch from photograph showing fault relations half a mile west of San erosion surface is displaced 7 feet by a large fault. Ysidro and 1,000 feet north of State Highway 44. Cretaceous rocks (Ku) faulted The formation of the higher surface therefore must down about 2,500 feet against Permian or Triassic (P"E). The base of the Santa Fe formation (Tsf) has been displaced 7 feet. have preceded the latest structural movements in the Basin and Range part of the field. Locally the Santa Fe formation lies nearly horizontal 3. Minor faults in the eastern part of the CoTorado on faulted and tilted Cretaceous formations, as at the Plateau portion of the volcanic field are parallel to the locality in the Antonio Sedillo grant shown in plate set of faults in the Basin and Range part of the field, 19, A and these minor faults locally displace the old sheet The Santa Fe does not extend westward into the basalts (p. 75). plateau part of the volcanic field, nor are the sheet The structural history of the Mount Taylor volcanic basalts known to extend eastward out of the plateau. field may, then, be summarized as follows: The north­ Indirect evidence, however, indicates that the sheet ward tilting of the southern San Juan Basin, the devel- 78 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1937 opment of the McCarty syncline, and probably also the westward thrusting of the Nacimiento further suggest folding and associated faulting northwest of the Mc­ active forces directed westward. Carty syncline and gentle warping in the eastern part The following facts collectively suggest that at least of the Colorado Plateau portion of the volcanic field had the stratified rocks in the Basin and Range portion of all been concluded before Mount Taylor erupted. This the Mount Taylor volcanic field were deformed by deformation ended probably before late Miocene time. horizontal tensional stresses or by stresses directed However, it began later than early Eocene time, because vertically: far to the north, in the interior of the San Juan Basin, 1. So far as known the faults are all normal faults the Wasatch formation was tilted northward. Probably on which there was very little horizontal movement the faulting in the Basin and Range part of the vol­ (p. 76). canic field began at about the same time, but this 2. The faults trend nearly parallel to the western faulting continued through later Tertiary time. The limit of the faulting. Experiments indicate that horizon­ eruptions of Mount Taylor and the later sheet basalts tal shearing would have developed en echelor faulting. were followed by minor folding and faulting within the 3. The width of the faulted area apparently has been McCarty syncline, south of Mount Taylor, minor expanded by faulting and diminished by folding and faulting in the gently warped area east of the syncline, tilting. To determine roughly the relative amounts and continued faulting and tilting in the Basin and of the expanding and shortening a computation was Range part of the volcanic field. These final movements made along a 6-mile line approximately coinciding with began in Pliocene time and probably continued into the south face of the block shown on plate 18, A. It Pleistocene time. was assumed that the dip of the fault surfaces continued POSSIBLE SIGNIFICANCE OF THE STRUCTURAL FEATURES. at depth as observed at the surface. The computation accordingly indicates an east-west shortening of 1,600 The presence in the southern San Juan Basin of feet by folding. The widening by faulting amounts to broad areas with gentle homoclinal dips implies the 2,800 feet. The net widening therefore would be 1,200 absence of active compressive stresses in the stratified feet in 6 miles, or nearly 4 percent. A second compu­ rocks. It is hardly conceivable that the weak strati­ tation was made along a line 5 miles in length about the fied upper crust of the southern San Juan Basin could position of the south face of the block shown in plate transmit stresses without considerable deformation. 18, B. This computation indicates a widening by Probably there was compression deep in the crystalline faulting of 1,000 feet and a shortening by folding of 250 basement, but deformation of the weak stratified feet, or a net widening of 750 feet, slightly less than 3 formations was apparently in the main an adjustment percent. It is fully conceded that these computations to irregularities developed in the upper part of the take cognizance only of those structural features that basement rocks. Renick13 suggests that the Nacimiento are recorded on a structure map having a scale of 1 mile uplift was thrust west because the basement rocks were to the inch and a 100-foot contour interval. Further­ uplifted sufficiently to allow their being pushed west­ more, the assumption of maintenance of dip by the fault ward against the weak stratified formations. The surfaces is probably false. No claim is made that the same interpretation may be applied to the linear figures even approach precision, but in the absence of folding in the Gallup-Zuni Basin, west of the Zufii other evidence they at least suggest that the area has uplift.14 been widened by a few percent. The stratified formations overlying an arch of base­ ment rocks would be stretched and fractured, probably Possibly the faulting resulted in part from deep by normal faulting. The close relationship of faulting folds arching the stratified rocks. Mesa Lucero and to folding in the northwestern part of the Mount Taylor the southeast flank of the San Juan Basin may be inter­ volcanic field has already been cited (p. 75) and per­ preted as the west flank of a broad arch whose axis is haps resulted from such a process. The deformation in the down-faulted Rio Grande Valley, and the Sandia of the southern San Juan Basin was at a maximum and Manzano Mountains as the east flank. But arch­ beneath the Mount Taylor volcano and diminished ing alone of the stratified rocks seems inadequate to progressively westward, suggesting that the assumed explain fully the abundant faulting. Back of the west­ deep stresses were active from the east. The asym­ erly forces that produced the Zuni uplift and the def­ metry of the Zufii and Nacimiento uplifts and the ormation beneath Mount Taylor there may have been fairly deep tension. However, it is more like^ that an i' Renick, B. C., Geology and ground-water resources of western Sandoval County, appearance of horizontal tension was produced by N. Mex.: U. S. Geol. Survey Water-Supply Paper 620, p. 71, 1931. 14 Sears, J. D., Geology and coal resources ol the Gallup-Zuni Basin, N. Mex.: shearing of the surface rocks at a high angle to the U. S. Geol. Survey Bull. 797, pi. 1, 1925. horizontal. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 18

Horizontal and vertical scale 5,000 O iO,000 Feet

A. BLOCK DIAGRAM SHOWING STRUCTURAL SURFACE OF PART OF ANTONIO SEDILLO GRANT AT SOUTHWEST CORNER OF THE AREA. Isometric projection; vertical scale is not exaggerated. Shading by W. H. Bradley. Ku, Cretaceous shale; Jm, Jurassic sandstone.

Horizontal and vertical scale

B BLOCK DIAGRAM SHOWING STRUCTURAL SURFACE OF MOST OF NORTH HALF OF BASIN AND RANGE PART OF MOUNT TAYLOR VOLCANIC FIELD. Isometric projection. Ku, Cretaceous shale and sandstone; Jm, Jurassic sandstone. GEOLOGICAL SURVEY PROFESSIONAL PAPER 189 PLATE 19

A. FLAT-LYING UNBROKEN SANTA FE FORMATION CROSSING A NORMAL FAULT IN CRETACEOUS SHALE. View looking north in Sedillo Grant 2 miles northeast of Suwanee. Tsf, Santa Fe formation; Ku, Cretaceous shale.

B. BASALTIC LAVA IN SANTA FE FORMATION TILTED AND BROKEN BY A SMALL FAULT. View along New Mexico State Highway 6 in sec. 2, T. 9 N., R. 1 W. Tsf, Santa Fe formation; Tb, basaltic lava.